H. Rodrigues Dias Filgueiras et al.: Wireless and Optical Convergent Access Technologies Toward 6G
provides higher SNR. However, the output power after pho-
tomixing is still low and most configurations require a elec-
tronic power amplifier operating in the THz range, before
the signal carrier can reach the transmitter antenna. Finally,
to establish a THz wireless link, the receiver can be based on
direct detection (typically Schottky or uni-traveling-carrier
photodiode (UTC-PD)) [54] or heterodyne detection, using
sub-harmonic diode mixers to obtain an IF frequency in the
mmWaves or microwave range [55].
There are other emerging photonic technologies capable
of generating THz waves. Among those, we can mention
quantum cascade laser (QCL) which operates emitting pho-
tons whose wavelength is dictated by electronic transitions
between conduction band energy-levels in a sequence of
quantum wells [48]. The operating principle results in photon
emission the high-frequency range, above 2 THz, and current
devices typically requires cryogenic operation in order to pro-
vide a useful output power, around a few hundred mW [56].
These drawbacks make very unlike that QCL-based THz
transmitters will be used in communications systems, at least
in the short and medium terms.
Another alternative would be to rely on the oscillation sur-
face plasmon-polariton (SPP) waves, particularly in graphene
or other two-dimensional (2D) materials such as molybde-
num disulfide (MoS2) [57]. This plasmonics-based approach
is very promising, particularly regarding the possibility of
small footprint and high frequency operation. Unfortunately,
this technology is still on its infancy, particularly concerning
output power and it is not likely to be mature enough for 6G.
Indeed, output power is also a limitation for photonic
systems based on photomixing. Even using state of the art
UTC-PDs for down-conversion, these system can typically
produce less than 10 mW at a few hundreds of GHz [58]. It is
then inevitable that even these photonics-based transceivers
will have to incorporate THz-electronics power amplifiers.
As a consequence, if that is the case, it is probably conve-
nient to approach the THz transceiver configuration from the
other end of the THz gap, namely, to seek an all-electronic
transceiver. As an additional advantage, such all-electronic
implementation would be much more familiar to the wireless
industry, thereby likely to facilitate future 6G deployments.
Although oscillators using two-terminal devices, such as
resonant tunneling diode (RTD), have already pushed the
1-THz boundary [59], most work on THz carrier genera-
tion by electronic means is based on the concatenation of a
chain of frequency multipliers, in such way to up-convert a
mmWaves carrier to the THz band. Early demonstrations of
such schemes have used radio-astronomy hardware on GaAs
technology [60]. Today, although Si-based technologies are
gaining ground, particularly on the basis of SiGe Heterojunc-
tion Bipolar Transistors [61], best results are achieved using
InP high electron mobility transistors (HEMT), as transmis-
sion powers of hundreds of mW at hundreds of GHz have
been already demonstrated [62], which are well above the
current results for photonic down-conversion. As the tech-
nology evolves and the maximum oscillation frequency of
TABLE 1. State-of-the-art on antenna for 5G towards 6G.
high-speed transistors has already surpassed 1 THz [63], the
short-term trends should favor all-electronic THz transceiver
configurations, due to the increasing availability of several
useful integrated circuits. For instance, an InP-based, higher
power density amplifier has been already demonstrated, with
a bandwidth as high as 235 GHz [64].
C. ANTENNA TEC HNOLOGIES FOR SUB-6 GHz AND
mmWaves
This sub-Section presents the state-of-the-art on antennas
and antenna arrays for sub-6 GHz and mmWaves, focusing
on disruptive technologies and application-oriented antenna
proposals for 5G that could also be applied to 6G systems.
Manuscripts published from 2016 to 2021 have been pre-
ferred for this literature review, as complied in Table 1.
A 28 GHz switched-beam slot antenna based on surface
PIN (S-PIN) diodes has been proposed for 5G systems in
2017 by Yashchyshyn et al. [65]. This research goal was to
achieve multiple beams and enable switching among them.
The designed reconfigurable structure was composed of
15 reconfigurable slots with embedded S-PIN diodes. In this
VOLUME 11, 2023 9237