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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