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1
Millimeter-wave and Terahertz Spectrum
for 6G Wireless
Shuchi Tripathi, Nithin V. Sabu, Abhishek K. Gupta, Harpreet S. Dhillon
Abstract—With the standardization of 5G, commercial mil-
limeter wave (mmWave) communications has become a reality
despite all the concerns about the unfavorable propagation
characteristics of these frequencies. Even though the 5G systems
are still being rolled out, it is argued that their gigabits per second
rates may fall short in supporting many emerging applications,
such as 3D gaming and extended reality. Such applications will
require several hundreds of gigabits per second to several terabits
per second data rates with low latency and high reliability, which
are expected to be the design goals of the next generation 6G
communications systems. Given the potential of terahertz (THz)
communications systems to provide such data rates over short
distances, they are widely regarded to be the next frontier for
the wireless communications research. The primary goal of this
chapter is to equip readers with sufficient background about
the mmWave and THz bands so that they are able to both
appreciate the necessity of using these bands for commercial
communications in the current wireless landscape and to reason
the key design considerations for the communications systems
operating in these bands. Towards this goal, this chapter provides
a unified treatment of these bands with particular emphasis
on their propagation characteristics, channel models, design
and implementation considerations, and potential applications to
6G wireless. A brief summary of the current standardization
activities related to the use of these bands for commercial
communications applications is also provided.
Index Terms—6G, Millimeter waves, Terahertz communication.
I. Background and Motivation
The standardization of 5G new radio (NR) was driven by
the diverse throughput, reliability, and latency requirements
of ever evolving ecosystem of applications that need to be
supported by the modern cellular networks. Within 5G, these
applications are categorized as enhanced mobile broadband
(eMBB), ultra-reliable low latency communication (URLLC),
and massive machine type communication (mMTC). Right
from the onset, it was clear that a one-size-fits all solution
may not work for all the applications because of which the
recent generations of cellular systems have explored the use
of advanced communications and networking techniques, such
Shuchi Tripathi, Nithin V. Sabu and Abhishek K. Gupta
are with the Indian Institute of Technology Kanpur, India, Email:
shuchi@iitk.ac.in,nithinvs@iitk.ac.in,gkrabhi@iitk.ac.in.
Harpreet S. Dhillon is with Wireless@VT, Bradley Department of
Electrical and Computer Engineering, Virginia Tech, Blacksburg, USA,
Email: hdhillon@vt.edu. This research is supported by the Science and
Engineering Research Board (DST, India) under the grant SRG/2019/001459.
This is a preprint of the chapter that will appear in [1]. Here are the complete
details of the chapter:
S. Tripathi, N. V. Sabu, A. K. Gupta, H. S. Dhillon, "Millimeter-wave and
Terahertz Spectrum for 6G Wireless", in 6G Mobile Wireless Networks. Y.
Wu, S. Singh, T. Taleb, A. Roy, H. S. Dhillon, M. R. Kanagarathinam, A. De,
eds. Springer, 2021.
as network densification through the use of small cells, smarter
scheduling, and multiple antenna systems for improved spectral
efficiency, just to name a few. Perhaps the most striking differ-
ence of 5G from the previous generations of cellular systems
is the acknowledgment that the classical sub-6GHz spectrum
is not going to be sufficient to support the requirements of
the emerging applications. The millimeter wave (mmWave)
spectrum naturally emerged as a potential solution. Although
these bands were earlier thought to be unsuitable for the mobile
operations due to their unfavorable propagation characteristics,
the modern device and antenna technologies made it feasible
to use them for commercial wireless applications [2]. As a
result, the 5G standards resulted in the birth of commercial
mmWave communication.
Now, as we look into the future, it is evident that we
are slowly moving towards applications, such as virtual and
augmented reality, ultra-HD video conferencing, 3D gaming,
and the use of wireless for brain machine interfaces, which will
put even more strict constraints on the throughput, reliability,
and latency requirements. With the advancement of device
fabrication methods, it is also reasonable to expect that the
nano-scale communications will see the light of the day
soon. With the recent success of mmWave communication,
it was quite natural for the researchers to start looking at the
other unexplored bands of the radio frequency (RF) spectrum,
primarily the terahertz (THz) band that lies above the mmWave
band. The THz waves with enormous bandwidth can be used in
many applications that require ultra-high data rates. This along
with the existing sub-6GHz and mmWave bands can help
us achieve the true potential of many emerging applications.
Further, owing to their small wavelength, they can also be used
for micro and nano-scale communication. In the past, the use
of THz bands was limited to imaging and sensing due to the
unavailability of feasible and efficient devices that can work on
these frequencies. However, with the recent advancements in
THz devices, THz communication is expected to play a pivotal
role in the upcoming generations of communication standards
[3].
The primary goal of this chapter is to equip readers with
sufficient background about the mmWave and THz bands so
that they are able to both appreciate the necessity of using
these bands for commercial communications in the current
wireless landscape and to reason the key design considerations
for the communications systems operating in these bands. This
is achieved through a systematic treatment of this topic starting
with a detailed discussion of the propagation characteristics at
these frequencies leading naturally to the discussion on chan-
nel models that capture these characteristics. Throughout this
arXiv:2102.10267v1 [cs.IT] 20 Feb 2021
2
discussion, we carefully compare and contrast the propagation
characteristics of these new bands with the better known sub-
6GHz cellular bands and explain how the key differences
manifest in the channel models. Building on this background,
we then explain the implications of these differences on the
design considerations for mmWave and THz communications
systems and their potential applications to 6G systems. The
chapter is concluded with a brief discussion about the current
standardization activities related to the use of these bands for
commercial communications.
II. Introduction to mmWave and THz Spectrum
Until the 4G cellular standard, the commercial (cellular)
communication was limited to the conventional bands up to
6GHz, which are now referred to as the sub-6GHz cellular
bands. However, there are many bands in the 6−300 GHz range
(with enormous bandwidths) that have been used for a variety
of non-cellular applications, such as satellite communications,
radio astronomy, remote sensing, radars, to name a few. Due to
recent advancement in antenna technology, it has now become
possible to use this spectrum for mobile communication as
well. The frequency band from 30 −300 GHz with the wave-
lengths ranging from 1to 10 mm is termed the mmWave band
and offers hundreds of times more bandwidth compared to the
sub-6GHz bands. Although higher penetration and blockage
losses are the major drawbacks of mmWave communication
systems, researchers have shown that the same effects are
helpful in mitigating interference in modern cellular systems,
which exhibit dense deployment of small cells. This naturally
results in a more aggressive frequency reuse and increased
data security due to higher directionality requirement at the
mmWave frequencies [4]. The mmWave frequencies from
about 24 GHz to about 100 GHz are already being explored
as a part of 5G standard. As we think ahead towards 6G and
beyond systems, researchers have also started exploring the
0.1−10 THz band, which is collectively referred to as the THz
band (with the lower end of this spectrum being obviously of
more interest for communications applications).
A. Need for the mmWave and THz Bands
It is well-known that the mobile data traffic has been
exponentially increasing for more than a decade and this trend
is expected to continue for the foreseeable future. With the
penetration of wireless IoT devices in new verticals, such as
supply chains, health care, transportation, and vehicular com-
munications, this trend is further expected to accentuate. It is
estimated that 9.5billion IoT devices are connected globally in
2019 [5]. The International Telecommunications Union (ITU)
has further estimated that the number of connected IoT devices
will rise to 38.6billion by 2025 and 50 billion by 2030 [6],
[7]. Handling this data deluge and the massive number of IoT
devices are two of the key design goals for 5G networks [8].
Three possible solutions to meet these demands are to develop
better signal processing techniques for improved spectral ef-
ficiency of the channel, the extreme densification of cellular
networks, and the use of additional spectrum [9], [10]. Various
advanced techniques, such as carrier aggregation, coordinated
multi-point processing, multi-antenna communications as well
as novel modulation techniques have already been explored
in the context of current cellular networks. The chances of
getting orders of improvement from these techniques are slim.
Likewise, network densification increases interference, which
places fundamental limits on the performance gains that can
be achieved with the addition of more base stations [11], [12].
The focus of this chapter is on the third solution, which is to
use higher frequency bands.
The amount of available spectrum at mmWave frequen-
cies is very large when compared to sub-6GHz frequencies
(∼50 −100 times). As the bandwidth appears in the pre-log
factor of the achievable data-rate, mmWave communication
can potentially achieve an order of magnitude higher data rate,
which made it attractive for inclusion in the 5G standards.
While 5G deployments are still in their infancy, emerging
applications such as extended reality may require terabits-per-
second (Tbps) links that may not be supported by the 5G
systems (since the contiguous available bandwidth is less than
10 GHz). This has created a lot of interest in exploring the
THz band to complement the sub-6GHz and mmWave bands
in 6G and beyond systems [13], [14].
B. What can mmWave and THz Frequencies Enable?
Larger bandwidths available in the mmWave spectrum make
multi-gigabit wireless communication feasible, thus opening
doors for many innovations [4]. For instance, the mmWave
frequencies can enable wireless backhaul connections between
outdoor base stations (BSs), which will reduce the land-
acquisition, installation and maintenance costs of the fiber-
optic cables, especially for ultra-dense networks (UDNs).
Further, it enables to transform current “wired” data cen-
ters to completely wireless data centers with data-servers
communicating over mmWave frequencies with the help of
highly-directed pencil-beams. Another potential application is
the in-boggy vehicle-to-vehicle (V2V) communication in high
mobility scenarios including bullet trains and airplanes where
mmWave communication systems together with sub-6GHz
systems have the potential of providing better data rates [15].
Further, the THz spectrum consists of bands with available
bandwidths of a few tens of GHz, which can support a data
rates in the range of Tbps. The communication at THz is
further aided by the integration of thousands of sub-millimeter
antennas and lower interference due to higher transmission
frequencies. It is therefore capable of supporting bandwidth-
hungry and low latency applications, such as virtual-reality
gaming and ultra-HD video conferencing. Other applications
that will benefit from the maturity of THz communications
include nano-machine communication, on-chip communica-
tions, internet of nano things (IoNT) [16], and intra-body
communication of nano-machines. It can also be combined
with bio-compatible and energy-efficient bio-nano-machines
communicating using chemical signals (molecules) [17]. Such
communication is termed molecular communication [18].
C. Available Spectrum
Due to the varying channel propagation characteristics and
frequency-specific atmospheric attenuation, researchers have
3
TABLE I
Available bands at the mmWave [9], [19] and THz spectrum [20].
Name Specific bands Remarks
26 GHz band 26.5−27.5GHz, 24.25 −26.5
GHz
Incumbent services: fixed link services, satellite Earth station services, and short-range de-
vices. Earth exploration satellites and space research expeditions, inter-satellites, backhaul,
TV broadcast distribution, fixed satellite Earth-to-space services and high altitude platform
station (HAPS) applications.
28 GHz band 27.5−29.5GHz, 26.5−27.5
GHz,
Proposed mobile communication. Incumbent services: Local multi-point distribution ser-
vice (LMDS), Earth-to-space fixed-satellite service and Earth stations in motion (ESIM)
application.
32 GHz band 31.0−31.3GHz, 31.8−33.4
GHz
Highlighted as a promising band. Incumbent services: HAPS applications, Inter-satellite
service (ISS) allocation.
40 GHz lower band 37.0−39.5GHz, 39.5−40.5
GHz
Incumbent services: Fixed and mobile satellite (space-to-Earth) and Earth exploration and
space research satellite (space-to-Earth and Earth-to-space) services, HAPS applications.
40 GHz upper band 40.5−43.5GHz Incumbent services: Fixed and mobile satellite (space-to-Earth), broadcasting satellite
services, mobile services, and radio astronomy.
50 GHz 45.5−50.2GHz, 47.2−47.5
GHz, 47.9−48.2GHz, 50.4−
52.6GHz
Incumbent services: Fixed non-geostationary satellite and international mobile telecommu-
nication (IMT) services, HAPS applications.
60 GHz lower band 57.0−64.0GHz Unlicensed operation for personal indoor services, device to device communication via
access and backhaul links in the ultra-dense network scenario.
60 GHz upper band 64.0−71.0GHz Upcoming generations of mobile standards with unlicensed status in UK and USA.
Incumbent services: The aeronautical and land mobile services.
70/80/90 GHz band 71.0−76.0GHz, 81.0−86.0
GHz, 92.0−95.0GHz
Fixed and broadcasting satellite services (space-to-Earth) services. Unlicensed operation for
wireless device to device and backhaul communication services in the ultra-dense network
scenario in the USA.
252 −296 GHz band 252 −275 GHz, 275 −296 GHz Early proposal for land mobile and fixed service. Suitable for outdoor usage.
306 −450 GHz band 306 −313 GHz, 318 −333 GHz,
356 −450 GHz
Early proposal for land mobile and fixed service. Suitable for short range indoor commu-
nication.
identified specific bands in mmWave/THz spectrum that are
particularly conducive for the communications applications.
In the world radiocommunication conference (WRC) 2015,
ITU released a list of proposed frequency bands in between
24 −86 GHz range for global usage [21]. The selection of
these bands was done based on a variety of factors, such as
channel propagation characteristics, incumbent services, global
agreements, and the availability of contiguous bandwidth.
WRC-2019 was focused on the conditions for the allocation of
high-frequency mmWave bands dedicated to the 5G systems.
A total of 17.25 GHz of spectrum had been identified [19].
For the implementation of future THz communication systems,
WRC 2019 has also identified a total of 160 GHz spectrum
in the THz band ranging between 252 to 450 GHz. A brief
description of these mmWave and THz bands are given in
Table-I.
Although mmWave and THz bands have a huge potential for
their usage in communication, there are significant challenges
in their commercial deployments. In particular, communi-
cation in these bands suffer from poor propagation charac-
teristics, higher penetration, blockage and scattering losses,
shorter coverage range, and a need for strong directionality in
transmission. These challenges have obstructed the inclusion
of mmWave and THz bands in standards and commercial
deployments until now. With the advancements in modern
antenna and device technologies, it is now becoming feasible
to use these bands for communications. However, there are still
various design issues that need to be addressed before they
can be deployed at a large scale [15], [22]. In this chapter,
we will discuss the propagation characteristics of these bands
in detail as well as the challenges involved in using them for
communications applications.
III. Propagation at the mmWave and THz Frequencies
A. Differences from the Communication in Conventional
Bands
The communication at mmWave/THz frequencies differs
significantly from the communication at conventional mi-
crowave frequencies. This is attributed to the following im-
portant factors.
1) Signal Blockage: The mmWave/THz signals have a
much higher susceptibility to blockages compared to the
signals at the lower frequencies. The mmWave/THz communi-
cation relies heavily on the availability of line of sight (LOS)
links due to very poor propagation characteristics of the non
line of sight (NLOS) links [23]. For instance, these signals
can be easily blocked by buildings, vehicles, humans, and even
foliage. A single blockage can lead to a loss of 20 −40 dB.
For example, the reflection loss due to glass for the mmWave
signal is 3−18 dB while that due to building material like
bricks is around 40−80 dB. Even the presence of a single tree
amounts to a foliage loss of 17−25 dB for the mmWave signals
[10], [15], [24], [25]. Moreover, the mmWave/THz signals also
suffer from the self-body blockage caused by the human users
which can itself cause an attenuation of around 20-35 dB [24].
These blockages can drastically reduce the signal strength and
may even result in a total outage. Therefore, it is of utmost
importance to find effective solution to avoid blockages and
quick handovers in case a link gets blocked. On the flip side,
blockages, including self-body blockages, may also reduce
interference, especially from the far off BSs [26]. Therefore,
it is crucial to accurately capture the effect of blockages
in the analytical and simulation models of mmWave/THz
communications systems.
4
2) High Directivity: The second important feature of
mmWave/THz communication is its high directivity. In order
to overcome the severe path loss at these high frequencies, it
is necessary to use a large number of antennas at the trans-
mitter and/or receiver side [23]. Fortunately, it is possible to
accommodate a large number of antennas in small form factors
because antennas at these frequencies are smaller than those
at traditional frequencies due to the smaller wavelengths. The
use of a large antenna array results in a highly directional com-
munication. High beamforming gain with small beamwidth
increases the signal strength of the serving links while re-
ducing the overall interference at the receivers. However, high
directionality also introduces the deafness problem and thus
higher latency. This latency occurs due to the longer beam
search process which is a key step to facilitate the directional
transmission and reception. This problem is aggravated in the
high mobility scenario because both the user as well as the
BSs suffer from excessive beam-training overhead. Therefore,
new random access protocols and adaptive array processing
algorithms are needed such that systems can adapt quickly in
the event of blocking and handover due to high mobility at
these frequencies [27].
3) Atmospheric Absorption: Electromagnetic (EM) waves
suffer from transmission losses when they travel through the
atmosphere due to their absorption by molecules of gaseous
atmospheric constituents including oxygen and water. These
losses are greater at certain frequencies, coinciding with the
mechanical resonant frequencies of the gas molecules [28]. In
mmWave and THz bands, the atmospheric loss is mainly due
to water and oxygen molecules in the atmosphere, however,
there is no prominent effect of atmospheric losses at the
microwave frequencies. These attenuations further limit the
distance mmWave/THz can travel and reduce their coverage
regions. Therefore, it is expected that the systems operating at
these frequencies will require much denser BS deployments.
B. Channel Measurement Efforts
Many measurement campaigns have been carried out to
understand the physical characteristics of the mmWave fre-
quency bands both in the indoor and outdoor settings. These
measurement campaigns have focused on the study of path-
loss, the spatial, angular, and temporal characteristics, the
ray-propagation mechanisms, the material penetration losses
and the effect of rain, snow and other attenuation losses
associated with different mmWave frequencies. See Table II
for a summary [29].
Likewise, in [46], measurements have been carried out to
characterize THz wireless links for both indoor and outdoor
environments. In the case of outdoor environments, [46]
showed that interference from unintentional NLOS paths could
limit the BER performance. The impact of weather on high
capacity THz links was discussed in [47]. The frequency
ranges which are suitable for THz communication have been
studied in [48], [49]. In [50], intra-wagon channel characteri-
zation at 60 GHz and 300 GHz are done using measurements,
simulations, and modeling.
C. Propagation at mmWave and THz Frequencies
We now discuss key propagation characteristics of the
mmWave and THz frequencies.
1) Atmospheric Attenuation: The atmospheric attenuation
is caused by the vibrating nature of gaseous molecules when
exposed to the radio signals. Molecules with sizes comparable
to the wavelength of EM waves excite when they interact with
the waves, and these excited molecules vibrate internally. As
a result of this vibration, a part of the propagating wave’s
energy is converted to kinetic energy. This conversion causes
loss in the signal strength [51]. The rate of absorption depends
upon the temperature, pressure, altitude and the operating
carrier frequency of the signal. At lower frequencies (sub-6
GHz), this attenuation is not significant. But, higher frequency
waves undergo significant attenuation since their wavelength
becomes comparable to the size of dust particles, wind, snow,
and gaseous constituents. The two major absorbing gases at
mmWave frequencies are oxygen (O2) and water vapor (H2O).
As seen in Fig. 1, the peaks of O2absorption losses are
observed at 60 GHz and 119 GHz which are associated with
a loss of 15 dB/km and 1.4dB/km, respectively. Similarly, the
peaks of H2Oabsorption losses are observed at 23 GHz, 183
GHz and 323 GHz, which are associated with a loss of 0.18
dB/km, 28.35 dB/km and 38.6dB/km, respectively. Similarly,
380 GHz, 450 GHz, 550 GHz, and 760 GHz frequency bands
also suffer a higher level of attenuation. However, for short-
distance transmission, the combined effects of these atmo-
spheric losses on mmWave signals is not significant [24]. THz
communication is even more prone to the atmospheric effects
in the outdoor environment. We can see that the spectrum
between 600 and 800 GHz suffers 100 to 200 dB/km attenu-
ation which is ≈10-20 dB over the distance of approximately
100 m [52]. The absorption process can be described with
the help of Beer-Lambert’s law which states that the amount
of radiation of frequency 𝑓that is able to propagate from
a transmitter to the receiver through the absorbing medium
(termed the transmittance of the environment) is defined as
[53]
𝜏(𝑟, 𝑓 )=𝑃𝑟𝑥 (𝑟 , 𝑓 )
𝑃𝑡 𝑥 (𝑓)=exp(−𝜅𝑎(𝑓)𝑟),(1)
where 𝑃𝑟 𝑥 (𝑟, 𝑓 )and 𝑃𝑡 𝑥 (𝑓)are the received and transmitter
power, and 𝑟is the distance between the transmitter and the
receiver. Here, 𝜅𝑎(𝑓)denotes the absorption coefficient of the
medium. The 𝜅𝑎(𝑓)is the sum of the individual absorption
coefficient of each gas constituent, which depends on its
density and type [51].
2) Rainfall Attenuation: The wavelengths of the mmWave
spectrum range between 1to 10 millimeters, whereas the
average size of a typical raindrop is also in the order of
a few millimeters. As a result, the mmWave signals are
more vulnerable to blockage by raindrops than conventional
microwave signals. The light rain (say, 2mm/hr) imposes a
maximum loss of 2.55 dB/km whereas the heavy rain (say,
50 mm/hr) imposes a maximum loss of 20 dB/km. In tropical
regions, a monsoon downpour at 150 mm/hr has a maximum
attenuation of 42 dB/km at frequencies over 60 GHz. However,
in the lower bands of the mmWave spectrum, such as the 28
5
TABLE II
MmWave channel measurements efforts for various environments.
Scenario/Environment Measurement efforts
Indoor settings such as office
room, office corridors, univer-
sity laboratory.
•Narrowband propagation characteristics of the signal, received power and bit error rate (BER) measurements [30].
•Measurements of the fading characteristics/distribution [31].
•Effects of frequency diversity on multi-path propagation [32].
•RMS delay spread measurement [33].
•Effects of transmitter and receiver heights on normalized received power for LOS and NLOS regions [34].
Outdoor settings such as uni-
versity campus, urban environ-
ments, streets, rural areas, nat-
ural environments, and grass-
lands.
•Comparison of the propagation mechanisms and fading statistics of the received signals [35].
•Channel impulse response, CDFs of received signal envelope and RMS delay spread [36].
•Effects of multi-path scattering over foliage attenuation, mean and standard deviation of the path-loss [37].
•Impacts of rain attenuation on the link availability and signal depolarization [38].
•The path loss exponents and mean RMS delay spread of LOS and NLOS paths [39] [40].
•Outdoor measurement over a distance of 5.8km at 120 GHz [41].
•Effects of ground reflections and human shadowing on LOS path-loss measurements [42], [43].
High-speed train (HST) chan-
nel propagation measurements
in the outdoor scenario.
•The reflection and scattering parameters for the materials of the deterministic and random objects present in the
HST environment. The verification of channel model in terms of path-loss, shadow-fading, power delay profile
and small-scale fading [44].
Outdoor to indoor (O2I) prop-
agation measurements. •Effects of outdoor to indoor penetration losses on the number of multi-path components, RMS delay spread, angular
spread and receiver beam-diversity [45].
10GHz
3cm
100
3mm
1THz
0.3mm
10
30 m
100
3 m
1000
0.3 m
Frequency
Wavelength
10-2
10-1
100
101
102
103
Attenuation(dB/Km-One way)
20o 1 atm
H2O
7.5g/m3
H2O
O2
O2
H2O
H2O
H2O
H2O
CO2
CO2
CO2
O2
H2O
Visibility 50m
Fog (0.1g/m3)
Excessive rain
(150mm/h)
Heavy rain
(25mm/h)
Drizzile
(0.25mm/h)
Terahertz
Millimeter Submillimeter
Fig. 1. Variation of atmospheric absorption due to various factors with respect
to frequencies in the band 10GHz-1000THz. This figure is reproduced using
data from [54].
GHz and 38 GHz bands, lower attenuations of around 7dB/km
are observed during heavy rainfall, which drops to 1.4dB for
the coverage range of up to 200 m. Thus by considering short-
range communications and lower bands of mmWave spectrum,
the effect of rainfall attenuation can be minimized [24].
3) Blockage:
(i) Foliage attenuation: The presence of vegetation can cause
further attenuation at mmWave/THz frequencies. The
severity of foliage attenuation depends on the carrier
frequency and the depth of vegetation. For example, the
foliage attenuation loss of 17 dB, 22 dB and 25 dB
are observed at 28 GHz, 60 GHz and 90 GHz carrier
frequencies, respectively [24].
(ii) Material penetration losses: The mmWave and higher
frequencies cannot propagate well through obstacles like
room furniture, doors and walls. For example, a high pen-
etration losses of 24.4dB and 45.1dB was observed at 28
GHz signal when penetrating through two walls and four
doors, respectively [24]. The higher penetration losses
limit the coverage region of the mmWave transmitter in
the indoor-to-outdoor and outdoor-to-indoor scenarios.
A LOS probability model can be used to incorporate the
effects of static blockages on the channel. This model assumes
that a link of distance 𝑑will be LOS with probability 𝑝L(𝑑)
and NLOS otherwise. The expressions of 𝑝L(𝑑)are usually
obtained empirically for different settings. For example, for the
urban macro-cell (UMa) scenario [55]
𝑝L(𝑑)=min 𝑑1
𝑑,11−𝑒−𝑑
𝑑2+𝑒−𝑑
𝑑2,
where 𝑑is the 2D distance in meters and 𝑑1and 𝑑2were
the fitting parameters equal to 18 m and 63 m, respectively.
The same model is also applicable for the urban micro-cell
(UMi) scenario, with 𝑑2=36 m. There are some variations
in the LOS probability expressions across different channel
measurement campaigns and environments. For example, the
LOS probability model developed by NYU [56] is
𝑝L(𝑑)=min 𝑑1
𝑑,11−𝑒−𝑑
𝑑2+𝑒−𝑑
𝑑22
.
where the fitting parameters 𝑑1and 𝑑2were equal to 20 m
and 160 m, respectively.
These empirical models can be justified theoretically. In
[57], a cellular network with random rectangular blockages
was considered where blockages were modelled using the
Boolean process and it was shown that LOS probability is
given as
𝑝L(𝑑)=𝑒−𝛽𝑑 ,where 𝛽=2𝜇(E[𝑊] + E[𝐿])
𝜋,
where 𝐿and 𝑊are the length and width of a typical rectan-
gular blockage and 𝜇is the density of blockages. A different
blockage model known as the LOS ball model was introduced
6
in [58] which assumes that all links inside a fixed ball of radius
𝑅𝐵are LOS, i.e.,
𝑝L(𝑑)=I(𝑑 < 𝑅𝐵),where 𝑅𝐵=√2𝜇E[𝐿]
𝜋,
which can also be used in the analysis of mmWave cellular
networks.
4) Human Shadowing and Self Blockage: As discussed
earlier, propagation at mmWave/THz frequencies can suffer
significant attenuation due to the presence of humans including
the self-blockage from the user equipment itself. In [59],
human body blockages were modeled using a Boolean model
in which humans are modeled as 3D cylinders with centers
forming a 2D Poisson point process (PPP). Their heights were
assumed to be normally distributed. In indoor environments,
human blockages have also been modeled as 2D circles of
fixed radius 𝑟with centers forming a PPP (𝜇) [60]. The LOS
probability for a link of length 𝑑in this case comes out to be
𝑝L=1−𝑒−𝜇(𝑟 𝑑+𝜋𝑟 2)
The self-blockage of a user can also be modeled using a 2D
cone of angle 𝛿(which is determined by the user equipment
width and user to equipment distance), such that all BSs falling
in this cone are assumed to be blocked [61].
5) Reflections and Scattering: Consider an EM wave im-
pinging on a surface. If the surface is smooth and electrically
larger than the wavelength of the wave, we see a single
reflection in a certain direction. The fraction of the incident
field that is reflected in the specular direction is denoted by the
reflection coefficient of the smooth surface, termed Γ𝑠, which
also accounts for the penetration loss. The reflected power is
thus
PR=PΓ2
𝑠,
where 𝑃is the power of incident wave. However, if the surface
is rough, the wave gets scattered into many directions in
addition to a reflected component in the specular direction.
This phenomenon is known as diffuse scattering [62], which is
also exhibited by the mmWave/THz signals. As discussed next
in detail, this behavior is attributed to the smaller wavelengths,
which are comparable to the size of small structural features
of the buildings surfaces.
Most importantly, whether a surface will be perceived
smooth or rough depends upon the incident wave’s properties.
The Rayleigh criteria can be used to determine the smooth-
ness or roughness of a surface based on the critical height
associated to the wave ℎ𝑐, which is given as [62]
ℎ𝑐=𝜆
8 cos 𝜃𝑖
,
where ℎ𝑐depends on the incident angle 𝜃𝑖and wavelength
𝜆. Let the minimum-to-maximum surface protuberance of
the given surface be denoted by ℎ0, while the RMS height
of the surface is ℎrms . Then, if ℎ0< ℎ𝑐, the surface can
be considered smooth, and if ℎ0> ℎ𝑐, the surface can be
considered rough for the particular wave with wavelength 𝜆.
This implies that as 𝜆decreases, the same surface which was
smooth at higher 𝜆, may start becoming rough. Therefore, at
Normal
Incident wave
Reflected wave
Scattered wave
Rough surface
𝜃𝑖
𝜃𝑟𝜃𝑠
Ψ
Fig. 2. Schematic diagram of a radio wave incident at a surface. 𝜃𝑖is the
incident angle, 𝜃𝑟is the reflected angle, 𝜃𝑠is the scattered angle and Ψis
the angle between reflected and scattered waves.
lower frequencies, reflection phenomenon is significant, while
scattering is negligible as most surfaces are smooth compared
to the wave. As a result, reflections are more prominent in
the lower mmWave bands while the scattering is moderate.
However, as we go higher in frequency to the THz bands,
scattering becomes significant since the roughness in the
surface of building walls and terrains becomes comparable
to the carrier wavelength. As a result, the scattered signal
components at THz are more significant compared to the
reflected paths.
For rough surfaces, scattering results in additional loss in
the reflected wave, if there is one. Therefore, the scattering
loss factor (denoted by 𝜌) has to be considered to obtain the
reflection coefficient Γof a rough surface [62]
Γ = 𝜌Γ𝑠,with 𝜌≈exp "−8𝜋ℎrms cos 𝜃𝑖
𝜆2#.
Therefore, the scattered power from this surface is given by
PS=P1−𝜌2Γ2
𝑠,(2)
and the reflected power is given by
PR=PΓ2=P𝜌2Γ2
𝑠.(3)
The fraction of the incident wave that is scattered is repre-
sented by scattering coefficient 𝑆2. The scattering coefficient
𝑆2is given by
𝑆2=PS
P=1−𝜌2Γ2
𝑠.
There are various models to characterize the variation of
scattering power with scattering direction. One of the widely
used models is the directive scattering (DS) model, which
states that the main scattering lobe is steered in the general
direction of the specular reflected wave (𝜃𝑟in Fig. 2) and the
scattered power in a direction 𝜃𝑠is
PS(𝜃𝑠) ∝ 1+cos(𝜃𝑠−𝜃𝑟)
2𝛼𝑅
,
where 𝛼𝑅represents the width of the scattering lobe. In [63],
DS model is used to model the propagation of a 60GHz wave
in the hospital room. The DS model was found to agree with
rural and suburban buildings scattering when validated with
1.29 GHz propagation measurements [64].
We can now compute the scattered power at a receiver from
a transmitter located at 𝑟𝑖distance away from the surface. From
7
the Friis equation and (2), the total scattered power from the
surface can be expressed as
PS=𝑆2𝐴𝑠
Pt𝐺𝑡
4𝜋𝑟2
𝑖
,
where Ptis the transmitted power, 𝐺𝑡is the transmitter antenna
gain, and 𝐴𝑠is the effective aperture of the scattering surface.
Now from DS model, the scattered power PSat a distance 𝑟𝑠
in the direction 𝜃𝑠, is
PS(𝜃𝑠)=PS0 1+cos(𝜃𝑠−𝜃𝑟)
2𝛼𝑅
where PS0 is the maximum scattered power given as
PS0 =PS
𝑟2
𝑠∫ ∫ 1+cos (𝜃𝑠−𝜃𝑟)
2𝛼𝑅
d𝜃𝑠d𝜙𝑠
If we define 𝐹𝛼=∫ ∫ 1+cos (𝜃𝑠−𝜃𝑟)
2𝛼𝑅
d𝜃𝑠d𝜙𝑠, then
PS0 =¯
PS
𝑟2
𝑠𝐹𝛼
=𝑆2𝐴𝑠
Pt𝐺𝑡
4𝜋𝑟2
𝑖
1
𝑟2
𝑠𝐹𝛼
.
Hence, the received power at the receiver located at an angle
𝜃𝑟and a distance 𝑟𝑠from the surface, is given as
pr=PS(𝜃𝑠) × Effective antenna aperture =PS
𝜆2
4𝜋𝐺𝑟
=𝑆2𝐴𝑠
Pt𝐺𝑡
4𝜋
1
𝑟2
𝑖𝑟2
𝑠
𝜆2
4𝜋𝐺𝑟
1
𝐹𝛼1+cos(𝜃𝑠−𝜃𝑟)
2𝛼𝑅
,(4)
where 𝐺𝑟is the receiver antenna gain. The model can also be
extended to consider the backscattered lobe.
6) Diffraction: Owing to its short wavelength, in
mmWave/THz frequencies, diffraction will not be as prominent
as it is at microwave frequencies [52]. In these frequencies,
NLOS has significantly less power compared to that of the
LOS path [65]. However, it may be possible to establish THz
links in the shadow of objects with the help of diffraction [66].
7) Doppler Spread: Since the Doppler spread is directly
proportional to the frequency and the speed of users, it is
significantly higher at mmWave frequencies than the sub-6
GHz frequencies. For example, the Doppler spread at 30 GHz
and 60 GHz is 10 and 20 times higher than at 3GHz [24].
8) Absorption Noise: Along with attenuation in the signal
power, molecular absorption causes the internal vibration in
the molecules which results in the emission of EM radiation at
the same frequency as that of the incident waves that provoked
this vibration. Due to this, molecular absorption introduces an
additional noise known as absorption noise. Since absorption
is significant in the THz bands, absorption noise is included in
the total noise as an additional term. It is generally modeled
using an equivalent noise temperature of the surroundings
caused by the molecular absorption [53].
9) Scintillation Effects: Scintillation refers to the rapid
fluctuation in the wave’s phase and amplitude due to the fast
local variation in the refractive index of the medium through
which the wave is travelling. Local variation in temperature,
pressure, or humidity causes small refractive index variations
across the wavefront of the beam which can destroy the phase
front, and the beam cross-section appears as a speckle pattern
with a substantial local and temporal intensity variation in
the receiver. Infrared (IR) wireless transmission distance is
limited by scintillation effects [67]. The result of scintillation
on practical THz communication is smaller than the IR beams.
The THz waves traveling close to the surface of the earth may
be influenced by the atmospheric turbulence [68]. However,
the extent to which scintillation effects impact the THz bands
is still not well understood.
D. Beamforming and Antenna Patterns
In multiple inputs and multiple outputs (MIMO) systems,
beamforming is used to focus a wireless signal towards a
specific receiver (or away from certain directions to avoid
interfering with devices in those directions). The gain thus
achieved in the signal to noise ratio (SNR) at the intended
receiver is called the beamforming gain, which is essential
in mmWave systems to ensure reliable reception. Traditional
MIMO systems were based on the digital beamforming, where
each element in the antenna array has its separate digital-to-
analog (D/A) conversion unit and the RF chain. However, fully
digital beamforming is not suitable for mmWave frequencies
due to many-fold increase in the number of antenna elements
which not only increases the cost of the overall system but
also the substantial power consumption [15]. Further the power
consumption generally scales linearly with the sampling rate
and exponentially with the number of bits per samples [10],
[15], [24], [25].
In order to lower the power consumption, analog beamform-
ing has been proposed for mmWave systems where a single
RF chain is shared by all antenna elements. Each antenna
is fed with the phase shifted version of the same transmit
signal where phase shift is determined according to the beam-
forming direction. However, such tranmission is limited to a
single stream and single user transmission/reception. To enable
multi-user/multi-stream transmission for mmWave networks
[10], [15], [24], [25], hybrid beamforming has been proposed
in which more than one RF chains are used. The hybrid
beamforming architectures are broadly classified into two
types, the fully connected hybrid beamforming architecture,
where each RF chain is connected to all antennas and the
partially connected hybrid beamforming architecture, where
each RF chain is connected to a subset of antenna elements.
Clearly, hybrid beamforming provides a tradeoff between low-
complexity but restrictive analog beamforming and the high-
complexity but most flexible fully digital beamforming.
1) Analog Beamforming Patterns: Due to analog beam-
forming, the effective gain in the received signal can be
computed using the transmitter and receiver antenna patterns
which represents the gain in different directions around the an-
tenna array (e.g., see (8)). Various antenna patterns have been
proposed in the literature to aid the evaluation of mmWave
systems. Some examples are discussed below.
Uniform linear array (ULA) model: For the antenna element
spacing 𝑑and signal wavelength 𝜆, the antenna gain of an 𝑁-
8
Fig. 3. The sectorized antenna model [15] which provides analytical tractabil-
ity in the system level evaluations of the mmWave systems.
array ULA [69] is
𝐺act (𝜙)=sin2(𝜋𝑁 𝜙)
𝑁2sin2(𝜋𝜙),(5)
where 𝜙=𝑑
𝜆cos 𝜃is the cosine direction corresponding to
the spatial angle of departure (AoD), 𝜃, of the transmit signal.
In order to avoid the grating lobes at mmWave frequencies,
the antenna element spacing 𝑑is generally kept to be half of
the wavelength. Since the spatial angle 𝜙depends on 𝑑, we
can use the approximation sin(𝜋𝜙) ' 𝜋𝜙 in the denominator.
Therefore the array gain function in (5) can be approximated
as a squared sinc-function
𝐺sinc (𝜙),sin2(𝜋𝑁 𝜙)
(𝜋𝑁 𝜙)2.(6)
This sinc antenna pattern has been widely used for the
numerical analysis in antenna theory. Authors in [70] have
verified the accuracy of tight lower bound provided by sinc
antenna model for the actual antenna pattern that makes it
highly suitable for the network performance analysis of the
mmWave systems.
Sectorized antenna model: To maintain the analytical
tractability in the network coverage analysis, many researchers
approximate the actual antenna pattern with the flat-top an-
tenna pattern, also known as the sectorized antenna model (see
Fig. 3). In this model, the array gains within the half-power
beam-width (HPBW) 𝜃3dB are approximated to the maximum
main-lobe gain 𝑔𝑚while the array gains corresponding to the
remaining AoDs are approximated to the first side-lobe gain
𝑔𝑠of the actual antenna pattern [58]. Hence, the gain in the
direction 𝜃is given as
𝐺Flat (𝜃)=(𝑔𝑚if 𝜃∈ [−𝜃3dB , 𝜃3dB]
𝑔𝑠otherwise.
Thus, the flat-top antenna pattern models the continuously
varying actual antenna array gains using the fixed main-
lobe and side-lobe gains. For highly dense network scenarios,
the aggregated interference from the side lobes is significant
because of which the term 𝑔𝑠must not be ignored in the
analysis. This model is limited in its ability to fit arbitrary
antenna patterns and is not suitable to analyze beam mis-
alignments.
Multi-lobe antenna model: In order to generalize the flat-
top antenna pattern, a multi-lobe antenna model was proposed
in [71] where there are 𝐾number of lobes, each with a
constant gain. The array gain and the width of each lobe are
obtained by minimizing the error function between the multi-
lobe pattern and the actual antenna pattern. The limitation
of the model includes the lack of the roll-off characteristic
of the actual antenna pattern because of which the predicted
analytical performance of the network may deviate from the
actual network performance [72].
Gaussian antenna model: The Gaussian antenna model is
proposed in order to capture the effects of roll-off in actual
antenna pattern which generally occur due to small perturba-
tions and misalignment between the receiver and transmitter
[73], [74]. The antenna gain for this model is given as
𝐺Gaussian (𝜃)=(𝑔𝑚−𝑔𝑠)𝑒−𝜂 𝜃2+𝑔𝑠.(7)
where 𝑔𝑚is the maximum main-lode gain which occurs as
𝜃=0,𝑔𝑠is the side-lobe gain and 𝜂is a parameter that
controls the 3dB beam-width.
Cosine antenna model: The antenna pattern for cosine
antenna pattern is given as [70]
𝐺cos (𝜃)=cos2𝜋𝑁
2𝜃 |𝜃| ≤ 1
𝑁.
This model can be extended to include multiple lobes [75] to
give additional flexibility.
2) Antenna Patterns for Multi-user/stream Transmission:
The above discussion can be extended to include the hybrid
beamforming supporting multi-stream or multi-user transmis-
sion. As a layered technique, hybrid beamforming can be seen
as linear combination of the digital and analog beamforming.
Hence the effective antenna pattern in case of multi user or
multi- stream transmission will consists of individual analog
beam patterns, one for each stream or user.
3) THz Beamforming: The limited transmission range of
THz waves can be extended somewhat via very dense ultra-
massive multiple-input multiple-output (UM-MIMO) antenna
systems. Since the number of antennas that can fit into the
same footprint increases with the square of the wavelength, the
THz systems can accommodate even larger number of antenna
elements than mmWave systems. This large array of compact
antennas results in highly focused beams (pencil beams) of
high gain that aids in increasing the transmission distance.
Similar to mmWave communication, the high cost and the
high power consumption in digital beamforming makes it
unsuitable for THz communication. The analog beamforming
at THz waveband can reduce the number of required phase
shifters in the RF domain. Nevertheless, it is subject to
the additional hardware constraints because the analog phase
shifters are digitally controlled and just have quantized phase
values, which will significantly restrict analog beamforming
performance in practice. On the other hand, the hybrid ana-
log/digital beamforming is again a better trade-off between the
analog and digital methods. The hybrid beamforming can have
fewer RF chains than antennas and approaches the fully digital
performance in sparse channels [76].
The types of antennas that can be used in THz commu-
nication are photoconductive antennas, horn antennas, lens
antennas, microstrip antennas, and on-chip antennas. Initially,
9
THz antennas were designed inside the semiconductor us-
ing Indium Phosphide (InP) or Gallium Arsenide (GaAs) in
which controlling radiation pattern was difficult due to high
dielectric constant. Therefore lens-based antennas that were
fed by horns were proposed. Other approaches, like stacking
different substrate layers with different dielectric properties,
were proposed to improve the antenna efficiency [77]. In
addition to metallic antennas and dielectric antennas, antennas
based on new materials are also possible e.g., carbon-nanotube
based antennas and the planar graphene antennas [78].
E. Channel Models
In order to evaluate the performance of the communication
system, the very first step is to construct an accurate channel
model. Not surprisingly, researchers have developed different
channel models for mmWave to be used in simulators and
analysis. For example, in 2012, the mobile and wireless com-
munications enablers for the twenty-twenty information society
(METIS) project proposed three-channel models, namely the
stochastic, the map-based and the hybrid model, where the
stochastic model is suitable for frequencies up to 70 GHz,
while the map-based model is applicable for frequencies up to
100 GHz. In 2017, 3GPP 3D channel model for the sub-100
GHz band was proposed. NYUSIM is another channel model
developed with the help of real-world propagation channel
measurements at mmWave frequencies ranging from 28 GHz
to 73 GHz in different outdoor scenarios [79]. Statistical
channel models for UM-MIMO are classified into matrix-
based models and reference-antenna-based models. Matrix-
based models characterize the properties of the complete
channel transfer matrix. On the other hand, the reference-
antenna-based models consider a reference transmitting and
receiving antenna first and analyze the point to point propa-
gation model between them. Then, based on this model, the
complete channel matrix is statistically generated [80].
As discussed above already, THz channels exhibit very
different propagation characteristics compared to the lower
frequency bands. Therefore, modeling the channel and noise is
essential for the accurate performance evaluation of the THz
communications systems [81]. Even the free-space scenario is
not straightforward to model in this case because of the signif-
icant level of molecular absorption. Therefore, one needs to be
include an additional exponential term along with the power-
law model in the path-loss equation. Overall, the peculiar
propagation characteristics of THz waves already discussed
in Section III-C make their analysis challenging.
Except for the recent measurements at the sub-THz fre-
quencies [52], the rest of the THz channel modeling work is
driven by ray tracing [82]–[84] or statistical channel modeling
[50], [85]–[91]. In particular, a statistical model for THz
channel based on a universal stochastic spatiotemporal model
has been introduced in [86] for indoor channels ranging from
275 GHz to 325 GHz. A 2D geometrical statistical model
for device-to-device scatter channels at sub-terahertz (sub-
THz) frequencies was proposed in [87], [88]. In addition to
the outdoor channel model, the indoor model for intra-wagon
channel characterization at 300 GHz was discussed in [50]. At
nano-scale level, the channel models were presented in [89],
[90] for intra body THz communication. A hybrid channel
model was discussed in [91] for chip-to-chip communication
via THz frequencies.
We now describe a simple yet powerful analytically tractable
channel model which can be adapted to various propagation
scenarios. This is suitable for the system-level performance
analysis including using ideas from stochastic geometry.
1) mmWave Channel: Consider a link between a transmitter
and a receiver located at 𝑟distance apart of type 𝑠where
𝑠∈ {L,N}denoting whether the link is LOS and NLOS [15].
For simplicity, let us assume narrow-band communication and
analog beamforming. The received power at the receiver is
given as
𝑃r=𝑃tℓ𝑠(𝑟)𝑔R(𝜃R)𝑔t(𝜃t)𝐻(8)
where
1) ℓ𝑠(𝑟)denotes the standard path loss at distance 𝑟which is
due to spreading loss. It is given by a path-loss function,
typically modelled using power-law as
ℓ𝑠(𝑟)=𝑐𝑠𝑟−𝛼𝑠,
where 𝑐𝑠is the near-field gain and 𝛼𝑠is the path-loss
exponent.
2) 𝑝tis the transmit power,
3) 𝑔tand 𝑔Rare the transmitter and receiver antenna pat-
terns while 𝜃tand 𝜃Rare the angles denoting beam-
direction of the transmitter and receiver. Therefore, 𝑔t(𝜃t)
and 𝑔R(𝜃R)are respectively the transmitter and receiver
antenna gains.
4) 𝐻denotes the small scale fading coefficient. Nakagami
fading is often assumed with different parameters 𝜇L
and 𝜇𝑁for LOS and NLOS link [15]. Therefore 𝐻is
a Gamma random variable with parameter 𝜇𝑠.
The above channel model can be extended for different
environments and propagation scenarios, for example, to in-
clude multiple paths [15], multi-rank channel [92], [93], hybrid
beam-forming [93] and massive MIMO. Since the specific
bands with high absorption loss are avoided, the effect of
molecular absorption can be ignored for mmWave commu-
nication.
2) THz Channel: Since atmospheric attenuation and scat-
tering are prominent at THz frequencies, the THz channel
model is expected to be different from the one discussed
above for the mmWave communications. Due to the huge
difference between LOS and NLOS links, most of the works
have considered LOS links only [94], [95]. For simplicity, we
will assume narrowband communication. If we consider a LOS
link between a transmitter and a receiver located at 𝑟distance
apart of type 𝑠, the received power 𝑃ris given by [46], [67]
𝑃r=𝑃tℓ(𝑟)𝑔R(𝜃R)𝑔t(𝜃t)𝜏(𝑟)(9)
where 𝜏(𝑟)is an additional loss term due to molecular
absorption defined in (1). In LOS links, path-loss can be given
by free space path loss i.e.,
ℓ(𝑟)=𝜆2
4𝜋1
4𝜋𝑟2.
10
The model can be extended to include scatters/reflectors. If
𝑟1is the distance between the transmitter and the surface while
𝑟2is the distance between the surface and the receiver, then
the scattered and reflected power are
Pr,S=𝑃t𝑔R(𝜃R)𝑔t(𝜃t)ℓ(𝑟1)𝑙(𝑟2)𝜏(𝑟1)𝜏(𝑟2)Γ𝑅
and
Pr,R=𝑃t𝑔R(𝜃R)𝑔t(𝜃t)ℓ(𝑟1+𝑟2)Γ2𝜏(𝑟1+𝑟2)Γ𝑆,
respectively, where Γ𝑅and Γ𝑆are coefficients related to re-
flection and scattering and may depend on surface orientation
and properties. The above channel model can be extended to
include other scenarios, for example, multiple paths and wide-
band communication [82].
IV. The mmWave Communications Systems
As discussed above already, the major advantage of us-
ing mmWave communications is the availability of abundant
spectrum, which is making multi-gigabit-per-second commu-
nication possible [96]. However, mmWave signals are more
susceptible to blockages and foliage losses, which necessi-
tates highly directional transmission. The combination of high
signal attenuation and directional transmission offers several
advantages and disadvantages for practical mmWave systems.
On the positive side, these make mmWave systems more
resilient to interference and hence more likely to operate in
the noise-limited regime [2]. Because of this, it is possible
for the operators to use higher frequency reuse factor, thereby
resulting in higher network capacity [97], [98]. For the same
reasons, mmWave transmissions are inherently more secure
compared to the sub-6GHz transmissions [99]–[102]. For
instance, the high attenuation of susceptibility to blockages
make it difficult for the remote eavesdroppers to even overhear
mmWave transmissions unless they are located very close to
the transmitters. Finally, as will be discussed in detail next,
these reasons also make spectrum sharing more feasible at the
mmWave frequencies.
On the flip side, with high directivity, the initial cell search
becomes a critical issue. Because of the use of directional
beams, both the BSs and users need to perform a spatial search
over a wide range of angles to align their transmission and
reception beams in the correct direction. This adds significant
delay and overhead to the communication. The situation de-
grades further when users are highly mobile due to increased
occurrences of handovers. Further, higher susceptibility to
blockages can result in outages. One approach to mitigate this
is to utilize the concept of macro-diversity [103], [104] and
[105], where simultaneous connections with multiple BSs are
maintained for each user so that it does not experience any
service interruption in the event of blocking of one BS.
After summarizing these key features of mmWave commu-
nications, we now discuss a few key implications of these fea-
tures on the system design. This section will be concluded with
a discussion on the potential uses of mmWave communications
in future 6G systems.
A. Key System Design Implications
1) Coexistence with lower frequency systems: Due to their
limited transmission range, a mmWave system may not work
effectively in a standalone deployment [106]. In particular, they
need to coexist with conventional cellular networks operating
on more favorable sub-6GHz bands such that all the control
level management, including load balancing and handovers,
is performed over sub-6GHz microwave transmissions while
the data transmissions occur over the mmWave bands. Such
networks will provide high capacity and better throughput
in comparison to the standalone networks without decreas-
ing reliability [107]. Further, macro-diversity can be utilized,
where multiple BSs (some can be sub-6 GHz and some are
mmWave) can connect to a user simultaneously to improve
LOS probability and link throughput [103].
2) Spectrum sharing: At lower frequencies, owning an
exclusive license of a spectrum band ensures reliability and
provides performance guarantees to applications with time-
critical operations for an operator. However, mmWave systems
often operate in a noise-limited regime because of which
exclusive licensing at these frequencies may result in an
under-utilization of the spectrum [9]. It has been shown that
spectrum sharing at mmWave frequencies does not require
sophisticated inter-cell coordination and even uncoordinated
spectrum sharing between two or more operators is feasible
[108]. This is an attractive option for the unlicensed spectrum
located at 59−64 GHz and 64−71 GHz bands which will allow
multiple users to access the spectrum without any explicit
coordination. Such a use of unlicensed spectrum increases
spectrum utilization and helps minimize the entry barrier for
new or small-scale operators. Even at licensed bands, shared
use of spectrum can help increase the spectrum utilization
and reduce licensing costs. It has also been shown that simple
inter-cell interference coordination mechanisms can be used
to improve the sharing performance [10], [15], [24], [25],
[109]. Furthermore, the bands where mmWave communication
coexists with other services (including incumbent services
and newly deployed applications) may need to protect each
other in the case of dense deployments. For this, spectrum
license sharing mechanisms such as uncoordinated, static, and
dynamic are the viable options in these bands. Spectrum
sharing opportunities at mmWave bands also bring the need
to evolve new methods of spectrum licensing which need to
be more flexible, opportunistic, dynamic and area specific [9].
3) Ultra-dense networks: Ultra-dense networks (UDN) are
characterized by very short inter-site distances. They are
generally used to provide local coverage in highly populated
residential areas, office buildings, university campuses, and
city centers. The mmWave frequencies are a natural candidate
for UDNs because of the directional transmission and blockage
sensitivity which limits interference even at ultra dense deploy-
ments. Further, self backhauling provides an inexpensive way
to connect these densely deployed APs/BSs to backhaul.
4) Deep learning-based beamforming: The performance of
the mmWave systems in a high mobility scenario is severely
affected by large training overhead, which occurs due to the
frequent updating of large array beamforming vectors. In the
last few years, deep learning-based beamforming techniques
11
have attracted considerable interest due to their ability to
reduce this training overhead. At the transmitter, pilot signals
from the UE are first transmitted to learn the RF signature of
the neighboring environment and then this knowledge is used
to predict the best beamforming vectors for the transmitted data
RF signature. Thus, after successful learning phase, the deep
learning models require negligible training overhead which
ensures reliable coverage and low latency for the mmWave
applications [110].
B. Potential Applications of mmWave Communications in 6G
1) Wireless access applications: Due to the abundance of
bandwidth around 60 GHz band, various technologies are
expected to be developed to support unlicensed operations
for the wireless local and personal area networks (WLANSs
and WPANs) with potential applications in internet access
at home, offices, transportation centers, and city hotspots.
These technologies are expected to support multi-gigabit
data transmission, with examples including IEEE 802.11ad
and IEEE 802.11ay [111], [112]. In future, IEEE 802.11ay
based mmWave distribution networks (mDNs) may become an
alternative-low-cost solution for the fixed optical fiber links.
The purpose of mDNs is to provide point-to-point (P2P)
and point-to-multi-point (P2MP) mmWave access in indoor
as well as outdoor scenario as well as wireless backhaul
services to the small cells in an ad-hoc network scenario.
The benefits of IEEE 802.11ay based mDN networks are
cheaper network infrastructure and the high-speed ubiquitous
coverage, while the major challenges include dealing with
blockages, interference management, and developing efficient
beam-training algorithms [113], [114]. Also, 5G is seen as
a significant step in enabling cellular communication over
mmWave bands which is expected to mature further in 6G
and beyond systems.
2) Backhaul infrastructure: It is well known that provid-
ing fiber backhaul in highly dense small cell deployments
is challenging due to increased installation and operational
cost [115]–[117]. Not surprisingly, many researchers have
recently invested their efforts to enable wireless backhaul in
mmWave bands owing to their directional communication and
high LOS throughput. The present 5G cellular backhaul net-
works are expected to operate on the 60 GHz and 71−86 GHz
bands, which are expected to be extended to the 92 −114.25
GHz band due to its similar propagation characteristics. Signif-
icant efforts have also gone into developing new technologies
including cross-polarization interference cancelation (XPIC),
bands and carriers aggregation (BCA), LOS MIMO, orbital
angular momentum (OAM) in order to increase the capacity of
the current mmWave backhaul solutions [118]. Further work
is needed to provide backhaul solutions for the data-hungry
future applications of 6G by using higher mmWave bands
(above 100 GHz) as well as the THz spectrum [118], [119].
3) Information showers: Information showers (ISs) are high
bandwidth ultra-short range hot spots in which mmWave BSs
operating at the unlicensed 60 GHz band are mounted on
the ceilings, doorways, entrances of the commercial buildings
or pavements, which deliver multi-gigabit data rates over a
coverage range of about 10 m [120]. Thus they provide an
ideal platform to exchange a huge amount of data between
different kind of networks, devices and users over a very short
span of time. Unlike conventional small-cell cellular networks,
ISs can be used for both offloading as well as pre-fetching
of data from the long haul wireless network for applications
like instant file transfer and video streaming. ISs also help in
improving the energy efficiency and battery life of the mobile
terminal due to its ability to download videos and large files
within a few seconds. However, installations of ISs requires
a very robust architecture that can work seamlessly with the
current cellular networks and is still an open area of research
[121], [122].
4) Aerial communications: Many frequency bands in the
mmWave spectrum region are already being used to support
the high-capacity satellite to ground transmission. However,
with the maturity of the drone technology, the future wireless
networks is expected to have a much more dynamic aerial
component with drones used in a diverse set of applications,
such as agriculture, mapping, traffic control, photography,
surveillance, package delivery, telemetry, and on-demand han-
dling of higher network loads in large public gatherings like
music concerts. Because of higher likelihood of LOS in many
of these applications, mmWave communications is expected to
play a particularly promising role. Further, quick (on-demand)
and easy deployment of drones also make them attractive
for many public safety applications, especially when the civil
communication infrastructure is compromised or damaged.
Naturally, mmWave communications can play a promising role
in such applications as well.
5) Vehicular communications: The ability of vehicles to
communicate among themselves as well as the wireless in-
frastructure not only helps in the navigation of completely
autonomous vehicles but is also helpful in the avoidance of
accidents in semi-autonomous and manually driven vehicles
through timely alerts and route guidance [123]. Because of
the high likelihood of LOS and the need to support high
data rates, the mmWave (and THz) spectrum is naturally
being considered for the vehicular communications systems
[39], [124]–[127]. Further, unified vehicular communications
and radar sensing mechanisms are needed for the massive
deployments of interconnected smart cars which can easily
cope with the rapidly maturing automotive environments,
consisting of the networked road signs, connected pedestrians,
video surveillance systems, and smart transportation facilities
[128].
V. The THz Communications Systems
After discussing mmWave communications systems in detail
in the previous section, we now focus on the THz communi-
cations systems in this section. Since the THz band is higher
in frequency than the mmWave band, the communication at
THz band faces almost all the critical challenges that we
discussed in the context of mmWave communications. In order
to avoid repetition, we will therefore focus on the challenges
and implications that are more unique (or at least more
pronounced) to the THz communications systems.
12
1) Smaller range: Due to high propagation and molecular
absorption losses, communication range of THz bands is
further limited compared to the mmWave transmission. For
instance, in small cells, the THz band may provide coverage
up to only about 10 m [129]. Further, the frequency-dependent
molecular absorption in the THz bands results in band-splitting
and bandwidth reduction [3].
2) THz transceiver design: In THz communication, the
transceivers need to be wideband, which is a major challenge.
The frequency band of the signal to be generated is too high for
conventional oscillators, while it is too low for optical photon
emitters. This problem is known as the THz gap. Another
challenge is the design of antennas and amplifiers which
support ultra-wideband transmission for THz communication
[81]. Currently, the THz waves are generated using either
conventional oscillators or optical photon emitters along with
frequency multiplier/divider.
3) THz beam tracking: Just like mmWave systems, the
THz communications systems require beamforming to over-
come large propagation losses. However, beamforming re-
quires channel state information, which is challenging to
obtain when the array sizes are large, as is the case in THz
communications systems. Therefore, it is vital to accurately
measure the AoD of transmitters and the angle of arrival (AoA)
of receivers using beam tracking techniques. While such beam
tracking techniques have been studied extensively for the lower
frequencies, it is not so for the THz frequencies. In THz
communication, in order to achieve beam alignment, beam
switching must be done before beam tracking. However, due
to large array sizes, the codebook design for beam switching
is computationally complex. On the flip side, these complex
codebooks will generate high-resolution beams, which help in
accurate angle estimation [76]. This provides a concrete exam-
ple of the type of challenges and subtle tradeoffs that need to
be carefully understood while designing THz communications
systems.
The implications of these challenges are similar to the
ones we discussed for the mmWave systems in the previous
section. For instance, due to the limited coverage area, the THz
communication systems are more likely to be deployed for
indoor applications [130]. In particular, the indoor links have
been found to be robust even in the presence of one or two
NLOS reflection components [46]. Likewise, owing to small
coverage areas and high directionality, it is expected that the
THz systems would efficiently share spectrum without much
coordination (similar to the mmWave systems). In bands where
passive services like radio astronomy and satellite-based earth
monitoring are already present, THz communication systems
need to share the spectrum under some protection rules.
A. Potential Applications of THz Communications in 6G
THz communication has many applications in macro as well
as in micro/nano scale. Some of the applications are discussed
in this section.
1) Macroscale THz Communication: Most of the
macroscale use cases of THz communications will be
driven by emerging applications requiring Tbps links, which
are not possible using mmWave spectrum. Such applications
include ultra HD video conferencing and streaming, 3D
gaming, extended reality, high-definition holographic video
conferencing, haptic communications, and tactile internet, to
name a few. Within conventional cellular network settings,
the THz bands are most suitable for small cell indoor
applications or high-speed wireless backhaul for small cells
[129]. Likewise, in the conventional WLAN applications,
the Terabit Wireless Local Area Networks (T-WLAN) can
provide seamless interconnection between high-speed wired
networks, such as optical fiber links, and personal devices,
such as mobile phones, laptops, and smart TVs. Along
similar lines, the Terabit Wireless Personal Area Networks
(T-WPAN) can enable ultra-high-speed communication among
proximate devices. A special type of WPAN application is
kiosk downloading, where a fixed kiosk download station is
used to transfer multimedia contents, such as large videos, to
mobile phones located in its proximity [131]. Other potential
applications and advantages of THz communications, such
as enhanced security, relevance for aerial and vehicular
communications [132], [133], as well as the potential use for
providing wireless connections in data centers, can be argued
along the same lines as we did already for the mmWave
networks in Section IV. In order to avoid repetition, we do
not discuss this again.
2) Micro/nanoscale THz Communication: The THz band
can also be used for enabling communications between
nanomachines [129]. These nanomachines can perform simple
tasks, such as computations, data storage, actuation, and sens-
ing. Depending on the application, the transmission distance
can vary from a few micrometers to a few meters. Some rep-
resentative applications of the nanomachine communications
are discussed below.
(i) Health monitoring: Nanosensors or nanomachines de-
ployed inside the human body can measure the level of
glucose, cholesterol, the concentration of various ions,
biomarkers emitted by the cancer cells, etc. [129]. The
measured data can be wirelessly transmitted to a device
outside the human body (e.g., mobile phone or a smart
band) using THz communication. The external device
can process the data and further send it to a medical
equipment or to a doctor.
(ii) Nuclear, biological and chemical defenses: Nanosensors
are capable of sensing harmful chemical and bio-weapon
molecules effectively [129]. In contrast to the classical
macro-scale chemical sensors, the nanosensors can detect
very small concentrations (as small as a single molecule).
As a result, the nano devices communicating in the
THz bands can be used in defense applications for the
detection of harmful chemical, biological and nuclear
agents.
(iv) Internet of nano-things (IoNT) and Internet of bio-nano-
things (IoBNT): The interconnection of nanomachines
with the existing communication network is known as
IoNT [16]. These interconnected nanodevices via IoNT
can serve a variety of purposes ranging from tracking
atmospheric conditions and health status to enabling real-
time tracking. Also, nano-transceivers and antennas can
13
be embedded in nearly all devices to be connected to the
Internet. IoBNT is conceptually the same as IoNT but
consists of biological nanomachines as opposed to the
silicon-based nanomachines [134]. Biological nanoma-
chines can be made from synthetic biological materials
and a modified cell via genetic engineering. IoBNT has
many applications in the biomedical field.
(iv) Wireless network on-chip communication: THz waves
can enable communication among processing cores em-
bedded on-chip with the help of planar nano-antennas of
a few micrometers in size [135]. This creates ultra-high-
speed inter-core communication for applications where
area is a constraint. Graphene based nano-antennas can
be used for the design of scalable and flexible wireless
networks on the chips.
B. Nanonetworks
While the capabilities of a single nanomachine are limited
to simple computations, sensing, and actuation, a network
of inter-connected nanomachines can perform much more
complex tasks. The nanomachines can communicate with each
other or with a central device. Such networks have a wide va-
riety of applications ranging from cancer treatment to environ-
mental monitoring. The two potential carriers of information
between nanomachines are EM waves and chemical molecules.
Inside the human body, molecular communication has several
advantages over EM waves, such as bio-compatibility and
energy efficiency.
Integration with molecular communication: A nano-scale
communication network consist of five fundamental compo-
nents [17]:
(i) Message carrier: Chemical molecules or waves that carry
information from the transmitter to the receiver.
(ii) Motion component: Provides force that is needed for the
message carrier to move in the communication medium.
(iii) Field component: Guides the message carrier in the com-
munication medium. External fields include the EM field,
molecular motors, and non-turbulent fluid flow. Internal
fields include swarm motion or flocking behavior.
(iv) Perturbation: This represents the variation of the message
carrier to represent the transmit information. Perturbation
is similar to modulation in telecommunication. It can
be achieved by varying the concentration or type of
molecules based on the transmit information.
(v) Specificity: Reception process of the message at the target.
For example, the binding of molecules with the receptor
structures present in the target.
A hybrid communication system that combines molecular
and EM paradigms was proposed in [17] (See Fig. 4). In
this hybrid communication network, MC is used inside the
body due to its bio-compatibility, energy efficiency, and the
lack of need for communication infrastructure for propagation
methods like diffusion-based MC (molecules propagates in
the medium based on concentration gradient). The nano-
nodes (bio-nanomachines) form clusters and sense the data
locally. A bio-nanomachine collects the health parameters,
modulates the data, and transmits the information to the other
Implantable nano-device
Nano-micro interface
THz communication
Implantable nano-device
Nanomachine
Information
Molecules
Molecular communication
Healthcare provider
Internet
Fig. 4. A hybrid nano communication network showing an eco-system
consisting of the biological and artificial components, where various com-
munication technologies including molecular and THz communication may
co-exist together [17].
bio-nanomachines (that act as relays). Now, for delivering
gathered information to a receiver outside the human body, a
graphene-based nano-device is implanted into the body. This
implantable nano-device is made up of a chemical nanosensor,
a transceiver, and the battery. Based on the concentration of
information molecules transmitted by the bio-nanomachines to
the implantable nano-device, the concentration is converted to
a corresponding electrical signal. Now the implantable nano-
devices communicate to the nano-micro interface via THz
waves. This interface can be a dermal display or a micro-
gateway to connect to the internet. This type of hybrid commu-
nication network is bio-compatible due to MC technology and
well connected to the outside world via THz communication.
VI. Standardization Efforts
We conclude this chapter by discussing the key standardiza-
tion efforts for both the mmWave and THz communications
systems.
A. Standardization Efforts for mmWave Communications
Given the increasing interest in mmWave communications
over the past decade, it is not surprising to note that several
industrial standards have been developed for its use. Some of
these standards are discussed below.
IEEE 802.11ad: This standard is focused on enabling
wireless communications in the 60 GHz band. It specifies
amendments to the 802.11 physical and MAC layers to support
multi-gigabit wireless applications in the 60 GHz band. The
unlicensed spectrum around 60 GHz has approximately 14
GHz bandwidth, which is divided into channels of 2.16,4.32,
6.48, and 8.64 GHz bandwidth. The IEEE 802.11ad standard
supports transmission rates of up to 8Gbps using single-input-
single-output (SISO) wireless transmissions over a single 2.16
GHz channel. It supports backward compatibility with existing
Wi-Fi standards in the 2.4and 5GHz bands. Therefore, future
handsets may have three transceivers operating at 2.4GHz
(for general use), 5GHz (for higher speed applications), and
60 GHz (for ultra-high-speed data applications) [107], [111],
[136].
IEEE 802.11ay: This standard is the enhancement of IEEE
802.11ad to support fixed point-to-point (P2P) and point-
to-multipoint (P2MP) ultra-high-speed indoor and outdoor
14
mmWave communications. It supports channel bonding and
aggregation to enable 100 Gbps data rate. Channel bonding
allows a single waveform to cover at least two or more
contiguous 2.16 GHz channels whereas channel aggregation
allows a separate waveform for each aggregated channel [112],
[137].
IEEE 802.15.3c: This standard defines the physical and
MAC layers for the indoor 60 GHz WPANs. In this standard,
the MAC implements a random channel access and time
division multiple access approaches to support the directional
and the quasi-omnidirectional transmissions [136].
ECMA-387: This standard proposed by the European com-
puter manufacturers association (ECMA) specifies the physical
layer, the MAC layer and the high-definition multimedia in-
terface (HDMI) protocol adaptation layer (PAL) for the 60
GHz wireless networks. The ECMA-387 standards can be
applied to the handheld devices used for low data rate transfer
at short distances and to the devices equipped with adaptive
antennas used for high data rate multimedia streaming at
longer distances [136].
5G NR mmWave standard: This is a global standard
platform for the 5G wireless air interfaces connecting mobile
devices to the 5G base stations. Together with the efforts of
IMT2020, the 3GPP Release 15 gave the first set of standards
detailing 5G NR use cases, broadly categorized as the eMBB,
the uRLLC and the mMTC. However, Release 15 is solely
dedicated to the non-standalone (NSA) operation of 5G NR in
which 4G LTE networks and 5G mobile technology co-exist
[138]. The 3GPP Release 16 (completed in July 2020) targeted
new enhancements for the better performance of standalone
(SA) 5G NR networks (operating in the range of 1−52.6GHz)
in terms of increased capacity, improved reliability, reduced
latency, better coverage, easier deployment, power require-
ments, and mobility. It also includes discussions on multi-
beam management, over the air (OTA) synchronization to
support multi-hop backhauling, integrated access and backhaul
(IAB) enhancements, remote interference management (RIM)
reference signals, UE power savings and mobility enhance-
ments [138]. The next release is expected to be completed in
September 2021, which will address the further enhancements
for the 5G NR. It will include the support for new services
like critical medical applications, NR broadcast, multicast and
multi SIM devices, mission critical applications, cyber security
applications, and dynamic spectrum sharing improvements, to
name a few [138].
B. Standardization Efforts for THz Communications
Given that the THz communications is still in its nascent
phase, its standardization efforts are just beginning. The IEEE
802.15.3d-2017 was proposed in 2017, which is the first
standard for THz fixed point-to-point links operating at carrier
frequencies between 252 and 321 GHz using eight different
channel bandwidths (2.16 GHz to 69.12 GHz and multiples
of 2.16 GHz). For the development of nano-network standards
at THz frequencies, IEEE P1906.1/Draft 1.0 discusses recom-
mended practices for nano-scale and molecular communication
frameworks [20], [139], [140].
VII. Conclusion
The main claim to fame for the 5G communications systems
is to demonstrate that mmWave frequencies can be efficiently
used for commercial wireless communications systems, which
until only a few years back was considered unrealistic because
of the unfavorable propagation characteristics of these frequen-
cies. Even though the 5G systems are still being rolled out, it
is argued that the gigabit-per-second rates to be supported by
the 5G mmWave systems may fall short in supporting many
emerging applications, such as 3D gaming and extended real-
ity. Such applications will require several hundreds of gigabits
per second to several terabits per second data rates with low
latency and high reliability, which are currently considered to
be the design goals for the next generation 6G communications
systems. Given the potential of THz communications systems
to provide such rates over short distances, they are currently
considered to be the next frontier for wireless communications
research. Given the importance of both mmWave and THz
bands in 6G and beyond systems, this chapter has provided
a unified treatment of these bands with particular emphasis
on their propagation characteristics, channel models, design
and implementation considerations, and potential applications.
The chapter was concluded with a brief survey of the current
standardization activities for these bands.
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