Page 21 - 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

as the conventional RF circuitry cannot support data proposed for improving user fairness as well as spectral
rates in the order of several 100s of Gbps or Tbps as ef iciency. For mitigating the water vapor‑based
provisioned in Section 3. Moreover, novel signal absorption effect in the THz band, the conventional
processing techniques will be required to counter the modulation schemes can be further optimized. For this
mismatch between the state‑of‑the‑art digital baseband purpose, in [51], distance‑aware multi‑carrier schemes are
systems and the large bandwidth offered by the THz proposed. Resource optimizations include power
band [52]. Recently, there have been advancements allocation as in [57], where long range networks are
related to the THz transceivers; both in the electronic established using a pulse‑based multi‑wide band waveform
and the photonic domains [22]. The advancements design by adapting power allocations over variable number
towards practical THz transceivers (both electronic of frames. By adapting the symbol time and modulation
and photonic) have been well summarized in [53]. order, in [59], a hierarchical modulation scheme is
For short‑range communication (below one meter), proposed for a system with a single transmitter and
impulse‑radio‑like communication based on one‑hundred‑ multiple receivers, supporting various streams of data for
femtosecond‑long pulses following an on‑off keying multiple users at variable ranges. In [62],
modulation spread in time has been proposed in [36]. distance‑adaptive and bandwidth‑dependent modulations
Such very short pulses, which are already utilized as using OFDM in THz band are proposed. It is worth
the basis of many THz sensing systems, can be mentioning here that, the aforementioned schemes have
generated and detected with current technologies. been proposed for THz band communications at sea level.
For longer communication distances, new dynamic band‑ These schemes will need to be adapted for speci ically the
width modulations are required for not only overcoming drone scenarios or DSNs, considering altitudes as well as
but also leveraging the unique distance‑dependent band‑ drone mobility.
width created by molecular absorption [11, 12]. Ortho-
gonal Frequency Division Multiplexing (OFDM) has Ultra‑Massive MIMO
widely been implemented in broadband wireless
systems since 4G for achieving higher spectral ef iciency. A main issue for the THz band communications is con‑
In [54], OFDM is proposed for 60 GHz millimeter wave stituted by the frequency‑selective and an extremely high
systems. For 5G, several wireless standards including path loss, which simply crosses 100 dB for ranges greater
Long Term Evolution (LTE), Wireless Fidelity (Wi‑Fi), than only a few meters under LOS channels. This path
Asynchronous Digital Subscriber Line (ADSL) etc., have loss is even worse under NLOS channel conditions. Con‑
adopted Cyclic Pre ix Orthogonal Frequency Division sequently, huge gains by highly directional antennas are
Multiplexing (CP‑OFDM)[55]. Multiple communicating required for communicating over ranges greater than a
nodes, each with Tbps of data (which would also be few meters. In this regard, the idea of Ultra‑Massive
the case in THz‑enabled DSNs) would require relaying (UM)‑MIMO has been proposed [63], where extremely‑
data in an asynchronous manner. Also, in such dense arrays of plasmonic nano‑antennas are employed.
asynchronous multiple user access, the subcarriers In lieu of deploying the traditional metallic antennas,
with CP‑OFDM do not remain orthogonal, which meta‑materials and nano‑materials can be exploited for
introduces substantial inter‑carrier interference [56]. manufacturing plasmonic nano‑antennas, which are suf‑
This makes CP‑OFDM infeasible for DSNs. Nevertheless, iciently less than the wavelength of the operating carrier
OFDM systems promise utilization of the non‑ frequency. This unique property of the plasmonic nano‑
overlapping spectrum for the improved spectrum antennas enables them to be packed in massively densed
ef iciency as compared to the pulse‑based arrays. For instance, for an array with a footprint of
communication systems [57, 58]. However, 1 mm x 1 mm, a sum of 1024 plasmonic nano‑antennas
implementation of OFDM transceivers in the THz band designed for 1 THz carrier frequency can be integrated
is especially complex due to stringent frequency together, keeping the inter‑element distance (spacing)
synchronization requirements, with the sampling rates of 1/2 of the plasmonic wavelength. Similar arrays of
in the typical order of several Giga samples/sec or the plasmonic nano‑antennas can be employed at both
even Tera samples per second. Additionally, large high the Tx and Rx sides simultaneously for countering the
Peak‑to‑Average Power Ratio (PAPR) also makes OFDM massive path loss issue by: 1) Overcoming the spread
implementation not feasible over the THz band [52, loss, by targeting the signal transmission in space, and
59]. In recent years, Non‑Orthogonal Multiple Access 2) focusing the bandwidths for the signal transmission
(NOMA) has gained considerable attention, as it within the windows having the least absorption levels.
promises not only greater link rates for both the down‑ By intelligently inputting the array elements, variations
link and uplink transmissions, but it also provisions a way of the modes of operation can be utilized in an
to counter the packet collision issue, e.g., in MTC with adaptive fashion. For instance, in UM‑Beam‑forming
grant‑free access [60]. NOMA is adopted for THz sys‑ (UM‑BF), all of the antennas utilize the identical
tems in [61] by making use of the frequency and distance‑ transmit signal, similar to the case of conventional
dependent THz spectrum. The concept of hybrid beam‑ beam‑forming. Such a mode can substantially counter
forming is proposed for forming user clusters, and NOMA‑ the massive path loss/attenuation at the THz band
based grouping and Long‑User‑Central‑Window (LUCW)‑ carrier frequencies, thus supporting communication to
are larger ranges. Additionally, beam‑forming also mitigates

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