Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond

Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond

Wireless communications above 100 GHz present significant opportunities and challenges for the development of 6G technology. This comprehensive analysis explores the potential applications of terahertz frequencies, including ultra-fast data transmission and advanced sensing capabilities. The paper discusses the technical hurdles that must be overcome, such as atmospheric attenuation and the need for innovative antenna designs. It also highlights recent regulatory advancements that pave the way for new wireless standards. Researchers and industry professionals interested in the future of wireless technology will find valuable insights and data in this document.

Key Points

  • Explores the potential of terahertz frequencies for 6G applications.
  • Discusses atmospheric attenuation challenges for wireless signals above 100 GHz.
  • Highlights recent regulatory advancements for wireless communication standards.
  • Analyzes innovative antenna designs necessary for effective terahertz communication.
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SPECIAL SECTION ON MILLIMETER-WAVE AND TERAHERTZ
PROPAGATION, CHANNEL MODELING AND APPLICATIONS
Received May 13, 2019, accepted May 27, 2019, date of publication June 6, 2019, date of current version June 28, 2019.
Digital Object Identifier 10.1109/ACCESS.2019.2921522
INVITED PAPER
Wireless Communications and Applications Above 100 GHz:
Opportunities and Challenges for 6G and Beyond
THEODORE S. RAPPAPORT
1
, (Fellow, IEEE), YUNCHOU XING
1
,
OJAS KANHERE
1
, SHIHAO JU
1
, ARJUNA MADANAYAKE
2
, (Member, IEEE),
SOUMYAJIT MANDAL
3
, (Senior Member, IEEE), AHMED ALKHATEEB
4
,
AND GEORGIOS C. TRICHOPOULOS
4
, (Member, IEEE)
1
NYU WIRELESS, Department of Electrical Engineering, Tandon School of Engineering, New York University, Brooklyn, NY 11220, USA
2
Florida International University, Miami, FL 33199, USA
3
Case School of Engineering, Case Western Reserve University, Cleveland, OH 44106, USA
4
Arizona State University, Tempe, AZ 85287, USA
Corresponding author: Yunchou Xing (yx775@nyu.edu)
This work was supported by the NYU WIRELESS Industrial Affiliates Program and the National Science Foundation (NSF) under Grant
1702967, Grant 1731290, Grant 1902283, Grant 1711395, Grant 1854798, and Grant 1730946.
ABSTRACT Frequencies from 100 GHz to 3 THz are promising bands for the next generation of wireless
communication systems because of the wide swaths of unused and unexplored spectrum. These frequencies
also offer the potential for revolutionary applications that will be made possible by new thinking, and
advances in devices, circuits, software, signal processing, and systems. This paper describes many of
the technical challenges and opportunities for wireless communication and sensing applications above
100 GHz, and presents a number of promising discoveries, novel approaches, and recent results that will
aid in the development and implementation of the sixth generation (6G) of wireless networks, and beyond.
This paper shows recent regulatory and standard body rulings that are anticipating wireless products and
services above 100 GHz and illustrates the viability of wireless cognition, hyper-accurate position location,
sensing, and imaging. This paper also presents approaches and results that show how long distance mobile
communications will be supported to above 800 GHz since the antenna gains are able to overcome air-
induced attenuation, and present methods that reduce the computational complexity and simplify the signal
processing used in adaptive antenna arrays, by exploiting the Special Theory of Relativity to create a cone of
silence in over-sampled antenna arrays that improve performance for digital phased array antennas. Also, new
results that give insights into power efficient beam steering algorithms, and new propagation and partition
loss models above 100 GHz are given, and promising imaging, array processing, and position location results
are presented. The implementation of spatial consistency at THz frequencies, an important component of
channel modeling that considers minute changes and correlations over space, is also discussed. This paper
offers the first in-depth look at the vast applications of THz wireless products and applications and provides
approaches for how to reduce power and increase performance across several problem domains, giving early
evidence that THz techniques are compelling and available for future wireless communications.
INDEX TERMS mmWave, millimeter wave, 5G, D-band, 6G, channel sounder, propagation measurements,
Terahertz (THz), array processing, imaging, scattering theory, cone of silence, digital phased arrays, digital
beamformer, signal processing for THz, position location, channel modeling, THz applications, wireless
cognition, network offloading.
I. INTRODUCTION
The tremendous funding and research efforts invested in
millimeter wave (mmWave) wireless communications, and
The associate editor coordinating the review of this manuscript and
approving it for publication was Thomas Kuerner.
the early success of 5G trials and testbeds across the world,
ensure that commercial wide-spread 5G wireless networks
will be realized by 2020 [1]. The use of mmWave in 5G
wireless communication will solve the spectrum shortage
in current 4G cellular communication systems that operate
VOLUME 7, 2019
2169-3536 2019 IEEE. Translations and content mining are permitted for academic research only.
Personal use is also permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
78729
T. S. Rappaport et al.: Wireless Communications and Applications Above 100 GHz
FIGURE 1. The electromagnetic spectrum, and various applications as a function of frequency.
at frequencies below 6 GHz [2]. However, the increasing
number of new applications such as virtual/augmented real-
ity (VR/AR), autonomous driving, Internet of Things (IoT),
and wireless backhaul (as a replacement for labor-intensive
installation of optical fiber) [3], [4], as well as newer appli-
cations that have not been conceived yet, will need even
greater data rates and less latency than what 5G networks will
offer.
Today, within the global unlicensed wireless mmWave
band of 60 GHz, there is ∼7 GHz of bandwidth avail-
able [5], and in such a wide bandwidth, data rates on the
order of 100 Gigabits per second (Gbps) can only be achieved
with transmission schemes having a spectral efficiency of at
least 14 bit/s/Hz, which requires symbol fidelity that is not
feasible using currently known digital modulation techniques
or transceiver components [6]–[8]. Therefore, data rates on
the order of 100 Gbps or more will flourish at frequencies
above 100 GHz, where the available spectrum is massively
abundant [9].
Fig. 1 illustrates the applications and range of frequencies
available from the sub-THz regime up through and beyond
the optical spectrum, and shows how mmWaves and THz
frequencies are three and two orders of magnitude, respec-
tively, below the frequencies of visible light. At optical and
infrared frequencies, issues like the impact of atmospheric
and water absorption on the signal propagation, ambient sun-
light, required low transmission power budget due to eye-
safety limits, and high diffusion losses on rough surfaces
limit their use for wireless communication systems [10].
Ionizing radiation, which includes ultraviolet, x-rays, galactic
radiation, and gamma-rays, is dangerous since it is known to
have sufficiently high particle energy to dislodge electrons
and create free-radicals that can lead to cancer [11], [12]
and is believed to be a major health risk for interplanetary
travel [12], [13]. The adverse health effects of ionizing radi-
ation may be negligible, however, if used with care [14].
Ionizing radiation can be used for gauging the thickness of
metals, Roentgen Stereophotogrammetry, astronomy, nuclear
medicine, sterilizing medical equipment, and pasteurizing
certain foods and spices [15]. Unlike ionizing radiation,
mmWave and THz radiation are nonionizing because the
photon energy is not nearly sufficient (0.1 to 12.4 meV,
which is more than three orders of magnitude weaker than
ionizing photon energy levels) to release an electron from
an atom or a molecule, where typically 12 eV is required
for ionization [11], [12], [16]. Since ionizing radiation is not
known to be a concern at mmWave and THz frequencies, and
heating is believed to be the only primary cancer risk [11],
[12], the Federal Communications Commission (FCC) and
International Commission on Non-Ionizing Radiation Protec-
tion (ICNIRP) standards [17], [18] are designed principally
to protect against thermal hazards, particularly for the eyes
and skin where these tissues are most sensitive to heat from
radiation due to lack of blood flow. However, we must point
out that with the likelihood of THz sources becoming more
widely available, there should be careful work done to under-
stand the biological and molecular impact of THz radiation
on human health [12], since, even though THz is more than
two orders of magnitude lower in frequency than ionizing
radiation, it would be prudent to know with certainty that
heating is the only health concern at THz [11].
While 5G, IEEE 802.11ay, and 802.15.3d [19], [20] are
being built out for the mmWave spectrum and promise data
rates up to 100 Gbps, future 6G networks and wireless appli-
78730 VOLUME 7, 2019
T. S. Rappaport et al.: Wireless Communications and Applications Above 100 GHz
cations are probably a decade away from implementation, and
are sure to benefit from operation in the 100 GHz to 1 THz
frequency bands where even greater data rates will be possi-
ble [3], [7], [10]. The short wavelengths at mmWave and THz
will allow massive spatial multiplexing in hub and back-
haul communications, as well as incredibly accurate sens-
ing, imaging, spectroscopy, and other applications described
subsequently in this paper [21]–[24]. The THz band, which
we shall describe as being from 100 GHz through 3 THz,
can also enable secure communications over highly sensitive
links, such as in the military due to the fact that extremely
small wavelengths (orders of microns) enable extremely high
gain antennas to be made in extremely small physical dimen-
sions [25]. Although we note the formal definition of the THz
region is 300 GHz through 3 THz, some have begun to use the
terms ‘‘sub-THz’’ or ‘‘sub-mmWave’’ (e.g. using frequency
or wavelength) to define the 100-300 GHz spectrum.
There are tremendous challenges ahead for creating com-
mercial transceivers at THz frequencies, but global research
is addressing the challenges. For example, the DARPA
T-MUSIC program is investigating SiGe HBT, CMOS/SOI
and BiCMOS circuit integration, in hopes of achieving power
amplifier threshold frequencies f
t
of 500-750 GHz [26].
A survey of power amplifier capabilities since the year
2000 is given in [27]. It should be clear that the semiconductor
industry will solve these challenges, although new architec-
tures for highly dense antenna arrays will be needed, due to
the small wavelengths and physical size of RF transistors
in relation to element spacing in THz arrays. Section III
provides some promising design approaches for future dig-
ital arrays.
Since there is very high atmospheric attenuation at THz
band frequencies, especially at frequencies above 800 GHz
(see Fig. 6), highly directional ‘‘pencil’’ beam antennas
(antenna arrays) will be used to compensate for the increased
path loss due to the fact that the gain and directivity increase
by the square of the frequency for a fixed physical antenna
aperture size [6], [28]. This feature makes THz signals
exceedingly difficult to intercept or eavesdrop [4], [10], [25],
[29]. However, a narrow pencil-like beam does not guaran-
tee immunity from eavesdropping, and physical-layer secu-
rity in THz wireless networks and transceiver designs that
incorporate new counter-measures for eavesdropping will be
needed [30].
Energy efficiency is always important for communication
systems, especially as circuitry moves up to above 100 GHz,
and a theoretical framework to quantify energy consump-
tion in the presence of vital device, system, and network
trade-offs was presented in [31], [32]. The theory, called the
consumption factor theory (CF, with a metric measured in
bps/W), provides a means for enabling quantitative analysis
and design approaches for understanding power trade-offs
in any communication system. It was shown in [31], [32]
that the efficiency of components of a transmitter closest
to the output, such as the antenna, have the largest impact
on CF [31]. The power efficiency increases with increasing
TABLE 1. Unlicensed spectrum proposed by FCC [33].
bandwidth when most of the power used by components that
are ‘‘off’’, e.g., ancillary, to the signal path (e.g., the baseband
processor, oscillator, or a display) is much greater than the
power consumed by the components that are in line with
the transmission signal path (e.g., power amplifier, mixer,
antenna) [31], [32]. For a very simple radio transmitter,
such as one that might be used in low cost IoT or ‘‘smart
dust’’applications where the power required by the ancillary
baseband processor and oscillator is small compared to the
delivered radiated power, the power efficiency is indepen-
dent of the bandwidth [31]. Thus, contrary to conventional
wisdom, the CF theory proves that for antennas with a fixed
physical aperture, it is more energy efficient to move up
to mmWave and THz frequencies which yield much wider
bandwidth and better power efficiency on a bits per second
per watt (bps/W) basis, as compared to the current, sub-6 GHz
communication networks.
Global regulatory bodies and standard agencies such as
the FCC [33], the European Telecommunication Standards
Institute (ETSI) [34], and the International Telecommuni-
cation Union (ITU) [35], are seeking comments to allocate
frequency bands above 95 GHz for point-to-point use, broad-
casting services, and other wireless transmission applications
and use cases [36]–[39]. In fact, in March 2019, the FCC
voted to open up spectrum above 95 GHz for the first time
ever in the USA, and provided 21.2 GHz of spectrum for
unlicensed use shown in Table 1, and permitted experimental
licensing up to 3 THz [40]. The mmWave coalition [41],
which is a group of innovative companies and universities
united in the objective of removing regulatory barriers to tech-
nologies using frequencies ranging from 95 GHz to 275 GHz
in the USA, submitted comments to the FCC and to the
National Telecommunications and Information Administra-
tion (NTIA) for developing a sustainable spectrum strategy
for America’s future, and urged NTIA to facilitate greater
access to spectrum above 95 GHz for non-Federal use in
January 2019 [41]. The Institute of Electrical and Electronics
Engineers (IEEE) formed the IEEE 802.15.3d [20] task force
in 2017 for global Wi-Fi use at frequencies from 252 GHz
to 325 GHz, creating the first worldwide wireless commu-
nications standard for the 250-350 GHz frequency range,
with a nominal PHY data rate of 100 Gbps and channel
bandwidths from 2 GHz to 70 GHz [20]. The use cases
for IEEE 802.15.3d include kiosk downloading (dubbed the
‘‘Information Shower’’ by an author of this paper) [42], intra-
device radio communication [43], connectivity in data cen-
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FAQs of Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond

What are the main applications of wireless communications above 100 GHz?
Wireless communications above 100 GHz enable revolutionary applications such as ultra-fast data transmission, advanced imaging, and precise localization. These applications are crucial for the development of 6G technology, which aims to support data rates exceeding 100 Gbps. The terahertz spectrum allows for innovative solutions in areas like autonomous vehicles, Internet of Things (IoT), and augmented reality. The paper emphasizes the significance of these applications in shaping future wireless networks.
What challenges are associated with terahertz communication?
Terahertz communication faces several challenges, primarily atmospheric attenuation, which can significantly impact signal propagation. As frequencies increase, the effects of rain, fog, and other environmental factors become more pronounced, necessitating the use of highly directional antennas to maintain signal integrity. Additionally, the development of cost-effective and power-efficient transceivers at these frequencies remains a critical hurdle. The document discusses these challenges in detail, providing insights into potential solutions.
How does the document address antenna design for 6G?
The document highlights the need for innovative antenna designs to effectively utilize the terahertz spectrum for 6G communications. It discusses concepts such as spatial noise-shaping and hybrid beamforming, which can enhance performance in high-frequency environments. The analysis includes recent advancements in antenna array technologies that are essential for overcoming the challenges posed by atmospheric attenuation. These design considerations are crucial for achieving reliable communication links in future wireless networks.
What regulatory advancements are mentioned for frequencies above 100 GHz?
Recent regulatory advancements have opened up new frequency bands above 100 GHz for wireless communications. The document outlines how bodies like the FCC are facilitating access to these bands, which is vital for the development of next-generation wireless technologies. These regulatory changes are expected to encourage innovation and investment in terahertz communications, paving the way for new applications and services. The paper emphasizes the importance of these advancements in shaping the future landscape of wireless communications.

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