Survey of Millimeter-Wave Propagation Measurements and Models in Indoor Environments

Abstract

The millimeter-wave (mmWave) is expected to deliver a huge bandwidth to address the future demands for higher data rate transmissions. However, one of the major challenges in the mmWave band is the increase in signal loss as the operating frequency increases. This has attracted several research interests both from academia and the industry for indoor and outdoor mmWave operations. This paper focuses on the works that have been carried out in the study of the mmWave channel measurement in indoor environments. A survey of the measurement techniques, prominent path loss models, analysis of path loss and delay spread for mmWave in different indoor environments is presented. This covers the mmWave frequencies from 28 GHz to 100 GHz that have been considered in the last two decades. In addition, the possible future trends for the mmWave indoor propagation studies and measurements have been discussed. These include the critical indoor environment, the roles of artificial intelligence, channel characterization for indoor devices, reconfigurable intelligent surfaces, and mmWave for 6G systems. This survey can help engineers and researchers to plan, design, and optimize reliable 5G wireless indoor networks. It will also motivate the researchers and engineering communities towards finding a better outcome in the future trends of the mmWave indoor wireless network for 6G systems and beyond.

Full text

electronics
Review
Survey of Millimeter-Wave Propagation Measurements
and Models in Indoor Environments
Ahmed Al-Saman 1,* , Michael Cheffena 1, Olakunle Elijah 2, Yousef A. Al-Gumaei 3,
Sharul Kamal Abdul Rahim 2and Tawfik Al-Hadhrami 4


Citation: Al-Saman, A.; Cheffena, M.;
Elijah, O.; Al-Gumaei, Y.A.; Abdul
Rahim, S.K.; Al-Hadhrami, T. Survey
of Millimeter-Wave Propagation
Measurements and Models in Indoor
Environments. Electronics 2021,10,
1653. https://doi.org/10.3390/
electronics10141653
Academic Editor: Ikmo Park
Received: 7 June 2021
Accepted: 8 July 2021
Published: 11 July 2021
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Attribution (CC BY) license (https://
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4.0/).
1Department of Manufacturing and Civil Engineering, Faculty of Engineering, Norwegian University of
Science and Technology (NTNU), 2815 Gjøvik, Norway; michael.cheffena@ntnu.no
2Wireless Communication Centre, School of Electrical Engineering, Faculty of Engineering,
Universiti Teknologi Malaysia, Johor Bahru 81310, Johor, Malaysia; elij_olak@yahoo.com (O.E.);
sharulkamal@utm.my (S.K.A.R.)
3Department of Computer and Information Science, Faculty of Engineering and Enviourement,
Northumbria University at Newcastle, Newcastle upon Tyne NE1 8ST, UK;
yousef.al-gumaei@northumbria.ac.uk
4School of Science and Technology, Nottingham Trent University, Nottingham NG11 8NS, UK;
tawfik.al-hadhrami@ntu.ac.uk
*Correspondence: ahmed.al-saman@ntnu.no
Abstract:
The millimeter-wave (mmWave) is expected to deliver a huge bandwidth to address
the future demands for higher data rate transmissions. However, one of the major challenges in
the mmWave band is the increase in signal loss as the operating frequency increases. This has
attracted several research interests both from academia and the industry for indoor and outdoor
mmWave operations. This paper focuses on the works that have been carried out in the study of the
mmWave channel measurement in indoor environments. A survey of the measurement techniques,
prominent path loss models, analysis of path loss and delay spread for mmWave in different indoor
environments is presented. This covers the mmWave frequencies from 28 GHz to 100 GHz that have
been considered in the last two decades. In addition, the possible future trends for the mmWave
indoor propagation studies and measurements have been discussed. These include the critical
indoor environment, the roles of artificial intelligence, channel characterization for indoor devices,
reconfigurable intelligent surfaces, and mmWave for 6G systems. This survey can help engineers and
researchers to plan, design, and optimize reliable 5G wireless indoor networks. It will also motivate
the researchers and engineering communities towards finding a better outcome in the future trends
of the mmWave indoor wireless network for 6G systems and beyond.
Keywords:
millimeter-wave propagation; radio channel; indoor environment; 28 GHz; 38 GHz;
40 GHz; 60 GHz; 70 GHz; wideband channel; 5G; 6G
1. Introduction
Wireless networks in an indoor environment are omnipresent, and their importance
can not be underestimated in our daily lives. Radio propagation study in wireless net-
works in realistic indoor environments leads us to build high-density networks with large
capacities. The frequency bands in the millimeter-wave (mmWave) spectrum
(30–300 GHz)
have gained increasing attention and now appear to be the most likely candidates to host
the upcoming wireless multi-gigabit applications for 5G wireless networks and beyond. In
this regard, the frequency range between 24.25 and 86 GHz for the future development of
International Mobile Telecommunications for 2020 and beyond has been proposed during
the 2015 World Radio Communication Conference [1].
The radio channel propagation can be affected by small scattering objects due to the
short wavelength of the utilized frequencies in the mmWave band. An indoor environment
represents the rich sources of scattering objects for radio channel propagation. There
Electronics 2021,10, 1653. https://doi.org/10.3390/electronics10141653 https://www.mdpi.com/journal/electronics

Electronics 2021,10, 1653 2 of 28
are many structural issues influencing the indoor radio channel propagation, such as
construction materials, building (size, rooms, corridors), number of people moving within
the room, a form of furniture and location and interaction with other systems. All of these
obstacles force the signal to propagate across multiple paths through reflections, refraction
and diffraction phenomena. Researchers and engineers have considered this problem since
the 1990s and developed myriad systems to provide channel models using path loss (PL),
and wideband parameters, i.e., resolved paths and delay resolution in many different
environments and frequencies.
The radio channel model can be classified into analytical models and physical mod-
els [
2
]. The analytic channel model is defined based on the mathematical analysis of the
channel. Analytical models characterize the communication channel mathematically and
can be derived from physical models such as the correlation-based Kronecker [
3
,
4
] and
Weichselberger [
5
] channel models as well as the propagation-based finite scatterer [
6
],
maximum entropy [
7
] and virtual channel representation [
8
] models, which are best used
for algorithm development and system analysis.
The physical channel models are constructed based on the double directional radio
channel between the transmitter (Tx) and Receiver (Rx) based on electromagnet wave
propagation. The physical channel models can be classified into deterministic, stochastic
and geometry-based stochastic. Deterministic channel models characterize the radio wave
propagation in a certain physical environment based on assumptions of the propagation
mechanisms. These models require a detailed geometry of the environment as well as
electromagnetic parameters of the materials. The radio wave propagation is calculated by
using ray-based methods or by solving Maxwell’s equations [
9
] with full-wave methods
such as the method of moments [
10
] or the finite difference time domain method [
11
]. The
full-wave methods have high accuracy but they are computationally very demanding. In
ray-based methods, rays are launched covering the full sphere (3D) or circle (2D) around
the Tx and traced until they reach the Rx, as proposed, e.g., in [
12
]. Stochastic models,
on the other hand, aim to describe the behaviour of the propagation channels statistically
without assuming the geometry of the environment. In stochastic modeling, the parameters
of the radio channel are defined by probability distribution functions, which can be tuned
based on channel measurements or deterministic modeling. An example of stochastic
models is the Saleh–Valenzuela model [
13
], which assumes that radio waves arrive to the
Rx in concentrated groups of multipath components, called clusters. In geometry-based
stochastic channel modeling (GSCM) the propagation channel from the Tx to the Rx is
characterized deterministically but the locations and properties of the scatterer objects are
chosen in a stochastic way. This approach has gained a lot of attention at lower microwave
frequencies and several reference models, such as COST273 and WINNER [
14
], are based
on GSCM. More details for physical radio channel models are found in [
15
]. Recently,
map-based mmWave channel models proposed such as a 3-D statistical model for 5G
wireless systems [
16
] and a backscattering channel model (Map-based) for personal radar
applications [
17
]. More details for map-based mmWave channel models can be found
in [18].
Surveys on mmWave have been identified in the literature [15,19–28]. These surveys
have discussed some of the key areas that border on propagation characteristics, channel
models, applications, mmWave technologies, design considerations such as environments,
scenarios, and operating frequency. While some of these works have discussed mea-
surement campaigns related to mmWave, there is still a lack of a comprehensive survey
of mmWave channel measurements in indoor environments. For instance, the authors
in [
24
,
28
] have presented the measurement techniques and the channel models for indoor
environments. There is a need for an up to date review of existing works and future
directions for the mmWave communication for indoor environments.
In this work, we provide an overview of the radio propagation study at mmWave
in indoor environments. Firstly, we reviewed several types of channel measurement
techniques in light of the different bandwidths of the probing signal. Secondly, we have

Electronics 2021,10, 1653 3 of 28
identified the mmWave frequencies that have been used over the last two decades in
indoor environments. Third, we present the path loss model for indoor environments
at mmWave bands. Fourth, the mmWave measurements result from different studies
in indoor environments are discussed. Finally, the future trends and research directions
are discussed.
2. Measurement Techniques
The most direct method of studying radio wave propagation is by channel measure-
ments, which will achieve statistical models that verify the propagation theoretical models.
Different measurement techniques with various experimental setups have been used to
study the different aspects of the radio frequency (RF) channels. In general, they can be
classified as narrowband (NB) and wideband (WB) techniques based on the relationship
between the probing signal bandwidth and the channel coherent bandwidth.
2.1. NB Measurement
NB measurement techniques are used to measure the path loss, narrowband fading,
small-scale fading characteristics and Doppler spread. The simplicity of this technique
is the main advantage of NB measurements. In narrowband techniques, a continuous
wave (CW) is transmitted and the received power is measured over space or time. The
basic block diagram of NB measurement is shown in Figure 1. The main drawback of
the NB measurement technique is that the received signal represents only the envelope
of the vector summation of the multipath components (MPCs); no quantitative multipath
information is available. Hence, if the bandwidth of the signal is high, the WB measurement
technique should be used to estimate the time dispersion parameters of the channel.
Figure 1. Narrow Band Measurement Block Diagram.
2.2. WB Measurement
The wideband channel sounders are used to resolve each MPC and provide the time
dispersion parameters. To extract the time dispersion parameters for the wideband channel,
the channel sounding measurements is applied. The wideband channel sounders are either
frequency domain (WBFD) or time domain (WBTD), where the Fourier transform is used
to convert between frequency and time domain.
In the frequency domain, the channel transfer function is measured using a vector
network analyzer (VNA), where the complex frequency response of the channel is measured
by the S
21
parameter. The block diagram for a VNA channel sounder-based system is
shown in Figure 2. Using the VNA system, the channel is measured at different frequency
tones along with the bandwidth of the system, by using stepped frequency sweeping.
Hence, the large bandwidth results in the slower measurement of the channel. So, the VNA
system can not be used to measure a time-variant channel, implying that it can be used
only for slow varying channels [24,29].

Electronics 2021,10, 1653 4 of 28
Figure 2. VNA Channel Sounder Frequency-domain Based System.
The time-domain measurement provides a more direct characterization approach.
These measurement techniques use a pulse generator to transmit short pulses of the order
of nano-seconds representing the large bandwidth of wideband channels, while a digital
sampling oscilloscope is used to record the received signal [
30
–
32
]. This measurement
technique is fast and suitable for rapid channel variations, however, it is a challenge to
generate short pulses with adequate power to achieve good-quality received signals.
A more prominent measurement technique in the time domain approach uses correla-
tion channel sounders (CS) [24,29]. In this method, a sequence of pulses such as a pseudo
noise (PN) sequence is sent by the Tx, while in the Rx part the cross correlation between
the transmitted and received signals is used to extract the channel. Since the Tx and the
Rx are separated in the time domain approach, it can be used for long distance, although
with a synchronization challenge. The correlation channel sounder approach is based on
the assumption that the clock rate of the pulses sequence is the same at the Tx and Rx for
real-time correlation processing. The wideband correlation channel sounder diagram is
shown in Figure 3. It supports the fast measurement speed, which needs more expensive
wideband digitizer. To use low cost narrowband digitizer with sacrifice in the measure-
ment speed, the sliding correlation channel sounder (SCS) has been widely used, which
approximates a true correlator receiver by multiplying the received signal with a “slow”
copy of the transmitted signal and then filtering the product [
33
,
34
]. The block diagram of
the SCS is shown in Figure 4. Although the hardware connection between the Tx and Rx is
not required by the SCS measurement technique, the separation PN generators at the Tx
and Rx require different references from the frequency oscillators. Moreover, the dynamic
range of the measurement is restricted by the clock rate of PN sequence differences.

Electronics 2021,10, 1653 5 of 28
Figure 3. Wideband Correlation Channel Sounder Time-domain Based System.
Figure 4. Sliding Correlation Channel Sounder Time-domain Based System.
3. Mmwave Indoor Channel Characteristics
In this section, we address indoor channel characteristics of mmWave based on path
loss and time dispersion.
3.1. Path Loss Models
The path loss represents the fundamental quantities characterizing the wireless propa-
gation channel and influencing the performance of any communication system. It is used to
characterize the wireless channels in terms of power decay with Tx-Rx separation distance.
Extensive studies have been done to investigate the indoor channel models for mmWave
in terms of path loss. Different path loss models have been proposed and investigated for
the indoor channels at different mmWave bands, including the close-in free space reference
distance (CI) model as well as the 3GPP and WINNER floating intercept (FI) model. The
path loss model estimates the amount of degradation on the propagated signals along
the propagation path with a certain distance for line-of-sight (LOS) and non-LOS (NLOS)

Electronics 2021,10, 1653 6 of 28
channels. The reflection, refraction, diffraction, and scattering are considered in path loss
evaluation. The CI path loss model is expressed as [35]:
PLC I (f,d)[dB] = PL(f,d0) + 10nlog10 d/d0+Xσ, (1)
where
PL(f,d)
is the path loss at different frequencies with various Tx-Rx separation
distance (
d
in meters),
PL(f
,
d0)
is the path loss at close-in reference distance
d0
in dB,
n
denotes the distance dependency of path loss and called path loss exponent (PLE), and
Xσ
is a zero-mean Gaussian distributed random variable with a standard deviation
σ
dB
(shadowing effects). The FI path loss model is defined as [35]:
PLF I (d)[dB] = α+10.βlog10(d)+XσF I , (2)
where
α
and
β
are the floating-intercept in dB and the slope of the line, respectively. The
shadow fading is represented by zero mean Gaussian random variable
XσFI
dB with
standard deviation of σdB.
Another popular model is the ABG path loss model, which is well-used to investigate
frequency dependency of path loss besides distance dependency in FI model. Recently, the
ABG model is widely used in mmWave band for 5G system and it is defined as [36]:
PLABG(f,d)[dB] =10αlog10(d)+β
+10γlog 10(f/1GHz) + XσA BG , (3)
where
α
is the distance dependence factor on path loss,
β
is an optimized offset,
γ
is the
frequency dependence factor and
XσABG
is the shadow fading term. The close-in free space
reference distance model with frequency-dependent PLE (CIF) is recently proposed to
model the propagation loss for indoor channels in the mmWave for the 5G system. It is
presented as [37]:
PLC I F =PL(f,d0) + 10n1+bf−f0
f0log10d/d0
+Xσ, (4)
where
n
denotes PLE, and
b
is an optimization parameter that captures the slope, or
linear frequency dependency of the PLE that balances at the centroid of the frequencies
being modeled. The term
f0
is a fixed reference frequency, the centroid of all frequencies
represented by the path loss model, found as the weighted sum of measurements from
different frequencies, using the following equation:
f0=∑K
k=1fkNK
∑K
k=1NK
, (5)
where
K
is the number of unique frequencies, and
Nk
is the number of path loss data points
correspondings to the
k
th frequency
fk
. The input parameter
f0
represents the weighted
frequencies of all measurement (or Ray-tracing) data applied to the model. For large indoor
distances (greater than 50 m), the breakpoint at a specific distance (dBp) was used and the
dual-slope ABG and CIF models were considered [38].
3.2. Time Dispersion Parameters
The root mean square delay spread (RMSDS) is used to characterize the time dispersion
properties of the wideband channel. It is an indicator of the possible extent of inter-symbol
interference according to the bandwidth of the signal [
39
]. It is defined as the second central
moment of the power delay profile (PDP) [40]:
τrmsds =qτ2−(¯
τ)2, (6)

Electronics 2021,10, 1653 7 of 28
where,
¯
τ=∑lP(τl)τl
∑lP(τl), (7)
τ2=∑lP(τl)τ2
l
∑lP(τl), (8)
The
(7)
and
(8)
represent the first moment (mean excess delay) and second moment of
the PDP, respectively. The P(τl)is the received power at lth multipath.
4. Review of mmWave Indoor Measurements
In this section, a review of published work on the mmWave frequencies that have
been used in different indoor environments is discussed.
The 30–300 GHz spectrum is assigned to the extremely high frequency (EHF) or
mmWave band, however, the industry often uses the term mmWave to define frequencies
between 10 GHz and 300 GHz [
41
]. Here, we will include the 28 GHz in mmWave band as
it is near the EHF spectrum and it has been used for many studies in indoor environments.
The 28 GHz was used for indoor environments recently (since 2013) for 5G wireless
networks. Many researchers have investigated radio propagation at mmWave band for
different scenarios in various indoor environments in both LOS and NLOS scenarios.
The measurements were conducted at different frequencies from 28 GHz to 100 GHz.
Different antennas with different gains and half-power beamwidths (HPBWs) in azimuth
(Az) or elevation (Ev) plane were used. Some of those antennas are directional such as
horn antennas or omnidirectional (Omni) such as biconical and open-ended waveguide
(OEW) antennas.
Tables 1–5give a comprehensive overview of published indoor mmWave channel
measurement campaigns in the last two decades from different regions in the world.
The tables describe the measurement scenario either LOS or NLOS or both LOS and
NLOS, measurement region, the measurement technique types, center frequency, the delay
resolution of wideband measurements that represented by inverse of radio frequency
bandwidth (
B
), the antenna type, gain, and HPBW, the Tx-Rx separation distance, and
the studied parameters. The reported indoor measurement campaigns in the literature
were taken in the indoor office, corridor, hall, different type of rooms (conference, meeting,
computer, class, living, dining, empty, and others), laboratory, shopping mall, library,
tunnel, courtyard, lobby, railway station, and airport.
Table 1. MmWave channel measurement in indoor offices.
Source Scenarios
LOS, NLOS Region Meas.
Type
Frq. (GHz)
/1
B(ns)
Antenna Type
/Gain (dBi)
AzHPBW (°)
/EvHPBW (°)
Distance
(m) Channel Statistics
[37] Both USA WBTD 28/2.5 Horn/20 15/15 4–46 PL, PDP,
RMSDS
[42] Both China WBTD 28/2 Horn/- 10/- - RMSDS, AOA, AS
[43] Both South Korea WBTD 28/4 Horn/24.5 10/10 10–40 PL,AS, RMSDS
[44] Both China WBTD 28/4 Omni - 1–45 PL, RMSDS
[45] Both China WBTD 28/- Horn 2- - PL
[46] Both China WBFD 28/1 and 0.5 Horn/19.2 20/18 <20 PL
[47] Both New Zealand WBTD 28/1.7 Biconical, Sectored,
Horn/2.4, 9.4, 25
360, 92.2,
and 10 <50 PL,RMSDS,
AS, K-factor
[48] Both - WBFD 28/1 Omni, Horn/6, 25 10/10 - PDP, AOA
[49] LOS China WBTD 28/2 Omni, Horn/8, 25 - 1–30 PL, RMSDS

Electronics 2021,10, 1653 8 of 28
Table 1. Cont.
Source Scenarios
LOS, NLOS Region Meas.
Type
Frq. (GHz)
/1
B(ns)
Antenna Type
/Gain (dBi)
AzHPBW (°)
/EvHPBW (°)
Distance
(m) Channel Statistics
[50] LOS South Korea WBTD 28/2 Horn/24.5 10 5–14 PL,AS,
RMSDS
[51] Both Denmark WBFD 26–30/0.25 horn/6, 19 20/20 5 - PL, PAPD
[52] LOS USA WBFD 26.5–40/0.07 Horn/10 -/55 0.2–1.8 PL
[53] - China NB 28 and 31 Horn - - Penetration loss
[54] Both South Korea WBTD 28 and 38/2 Omni/5 and 6 - 6–54 PL
[55] LOS UK WBTD 32 and 39 /0.5 Horn/10 54/54 4–20 PL, RMSD
[56] LOS Japan NB 37.1 Horn, Omni/- -/60 5–45 PL
[57] Both China NB 45 Horn,OEW,
VDA /23.7, 6, 2.5
12, 60, 120
/12, 60, 240 1–10 PL
shadow fading
[29] Both Germany WBTD 60/1, 0.2 Omni - 0.5–10 PL, RMSDS
[58,59] Both USA WBTD 60/2.5 Horn/25 50/50 3.5–24.7 PL, RMSDS
[60,61] Both Germany WBTD 60, 70/0.25 Horn and Lens/ 20 15/15 - AOD, RMSD,
PDP
[62] Both Japan WBFD 70/0.33 Horn, Omni/-/- 15,30,60/- 4–19 PL
[37,63,64] Both USA WBTD 73.5/2.5 Horn/20 15/15 4–42 PL, PDP,
RMSDS
[65,66] LOS Finland WBFD 61–65, 69–74
/0.25, 0.2 Omni, Horn/5, 20 20/20 0.9–10.3 Scattering
[67] Both China WBTD 60/0.5 Horn/25 10/10 2–6 AS, PDP,
PAS
[68] Both Austria WBFD 55–65/0.1 OEW/5 - 2.5–3 CIR
[69,70] LOS France WBFD 60/0.5 Omni, Horn/ 2, 22.5 13/10 - PL, CIR
4.1. Indoor Office
The office environment is characterized by obstructions from different objects such
as desks, chairs, cubicle partitions, doors, windows, and walls. Measurement set-ups are
positioned at heights above ground levels (AGL) such as 1.80 m [
45
] and 1.48 m [
46
]. A
review of the works in indoor offices environment at different frequencies are presented
as follows.
Based on directional measurements at 28 GHz using horn antennas in indoor offices
environments [
37
] the path loss (PL) results showed that the PLE values are 1.7 and 4.4
for LOS and NLOS scenarios, respectively. It was observed that when the polarization
of the Tx and Rx antennas are different (cross-polarization) such as vertical-horizontal
(V-H), the PLEs for the same particular scenario are 4.1 and 5.1 for LOS and NLOS sce-
narios, respectively. This implies that the received power significantly degrades when
the polarization of the Rx antenna is different from the Tx antenna polarization. For the
directional channel characteristics using horn antennas at Tx and Rx, the AzHPBW and
EvHPBW of the antennas are crucial for power delay profile (PDP) and the PL statistics.
Hence, to get the accurate channel statistics for such an environment using horn antennas
that have high gains, the horn antennas are usually installed on a rotational platform to
collect the signals from different directions. The Tx and Rx horn antennas should be rotated
to different angles where the total received power obtained by summing the received
powers at each steering direction [
37
] Large scale omnidirectional path loss models were
developed based on this concept as presented in [
37
] for both LOS and NLOS scenarios.
It was shown that the omnidirectional PLE values at 28 GHz are 1.1 and 2.7 for LOS and
NLOS scenarios, respectively. This implies that the received power significantly improved
for both scenarios when the rotation platform was used. The same results were reported
in [
46
,
50
] the omnidirectional PLE values based on directional measurements with rotation

Electronics 2021,10, 1653 9 of 28
platform are 1.6 and 2.1 for LOS and NLOS scenarios, respectively. Based on measurements
using omnidirectional antennas at both Tx and Rx [
54
] it was found that the PLE values
are 1.8 and 3.0 for LOS and NLOS scenarios, respectively. Some of the measurements used
omnidirectional antenna in one side only either in Tx or Rx and used a horn antenna on the
other side. In [
49
], it was found that the PLE for the LOS scenario at 28 GHz is 1.7 using
omnidirectional antenna at Tx and horn antenna at Rx. The same observations for the
effects of antenna types were found at 60 GHz band and 70 GHz band in different indoor
office environments.
Table 2. MmWave channel measurement in different rooms and halls.
Source Scenarios
LOS, NLOS Region Meas.
Type
Frq. (GHz)
/1
B(ns)
Antenna Type
/Gain (dBi)
AzHPBW (°)
/EvHPBW (°)
Distance
(m) Channel Statistics
[71]Dining room
LOS Malaysia WBTD 28, 38/1 omni, horn
/3, 19.2, 21.1 18.6, 14.5/- 1–10 PL, RMSDS
[72] Rooms (NLOS) USA, Chile NB 28/- Horn/10, 24 55, 10/- <100 PL
[73]Conference room
Both China WBFD 28/- Horn/20 17/14.6 1–10 PL
[74]
[51]
Empty room
Both Denmark WBFD
28–30/0.50
26–30/0.25
Biconical,
horn/6, 19 20/20
5
-
PL, PAPD
[75]Computer room
Both China WBTD 30, 60
/0.45, 0.9 Horn/15, 20 35 28 PDP, RMSDS
[57]Conference,
living rooms (Both) China NB 45/- Horn,OEW,
VDA /23.7, 6, 2.5
12, 60, 120
/12, 60, 240 1–10 PL
shadow fading
[76] Classrooms (Both) Malaysia NB 40/- Horn, Omni
/21.34, 3 - 1–27 PL
[77]Conference room
LOS China WBTD 39/0.5 Horn, Omni - 2–10 PL, RMSDS,
K-factor
[78]Conference room
Both China WBFD 45/1.5 Horn/18.9, 23.7 11, 18.2 - PDP, AOA
[79] Rooms (Both) USA WB 60/10 OEW, horn/6.7, 29 90, 7 2.4–60 PL, RMSDS,
PAP
[80]Conference room
LOS Germany WBTD 60/1 Omni/2 -/70 1–5 PL, RMSDS,
PDP
[81,82]Rooms (Both) Russia WBTD 60/1.3 Horn/16, 18 25, 20 <3 Polarization
[83]Computer room
LOS France LOS WBFD 60/0.5 Polarized/12, 10 30/30 1–7 PL, Fading
[84] Room (Both) Netherlands WBFD 60/0.5 Biconical/9/- 9 0.7–7 PL
[85]Empty room
LOS USA WBFD 60/2.5 bow-tie - - RMSDS, fading
[86]Conference room
Both Finland WBFD 61–65/0.25 Biconical, OEW - - AOD, AOA
[87] Room (Both) Netherlands WBFD 58/0.5 Omni /- - 0.5–14 PL, RMSDS
[88] Rooms (Both) Japan WBFD 61–65/0.25 Bioconical, OEW/2, 7 - 1–5 PL, RMSDS
[89]Conference room
LOS Germany WBTD 60/0.33 Omni, OEW/2, 8 - 2–10 Human body
shadowing effects
[69,70]Meeting room
LOS France WBFD 60/0.5 Omni, Horn/ 2, 22.5 13/10 - PL, CIR
[90] Rooms (Both) UK WBFD 57–64/1 Omni/6 - - PDP ,RMSDS
[91,92]Class rooms,
hall (Both) Denmark WBFD 28/0.5 Bicone/4.8, 6 - 2.5–6.5 PL,AS,
RMSDS
[93] Halls (Both) UAE NB 28/- Horn/15 - 1–60 PL

Electronics 2021,10, 1653 10 of 28
Table 3. MmWave channel measurement in indoor corridors/ hallways.
Source Scenarios
LOS, NLOS Region Meas.
Type
Frq. (GHz)
/1
B(ns)
Antenna Type
/Gain (dBi)
AzHPBW (°)
/EvHPBW (°)
Distance
(m) Channel Statistics
[35,36,94] Both Malaysia WBTD 28, 38/1 Omni, horn
/3, 11.6, 15.2
37.6, 27.5
/44.8, 28.3 1–67 PL, AS,
RMSDS
[72] NLOS USA,
Chile NB 28/- Horn/10, 24 55, 10/- <100 PL
[46] Both China WBFD 28/1 and 0.5 Horn/19.2 20/18 <20 PL
[95] Both China WBTD 28/1 Omni/5 - 3–20 PL, RMSDS,
K-factor
[93] LOS UAE NB 28/- Horn/15 - 1–60 PL
[96,97] LOS USA NB 30/- Omni - 1–6.5
8-13.5 PL
[98] Both USA NB 31 Horn/10 54/54 1–67,
18–25 PL
[99] Both Germany WBTD 30/- Horn/- 30 10–80 PL, RMSDS,
PDP
[76] Both Malaysia NB 40/- Horn, Omni
/21.34, 3 - 1–27 PL
[100–102] Both Spain WBFD 30, 39, and
40/0.026 Omni - 0.5–13 PL, K-factor,
RMSDS
[103] Both China WBTD 41/0.5 Horn/24 7/7 1.35–70 PL, RMSDS
[104] LOS Greece WBTD 60/8 Horn/21 36/11 - RMSDS
[105] LOS Germany WBTD 60/1 OEW, lens 6/120/- - PAP, PDP,
K-factor, RMSDS
[90] - UK WBFD 57–64/1 Omni, horn
/6, 10,10 -,69, 55/6.5,-,- - PDP , RMSDS
[99,106] Both Germany WBTD 60 and 74/- Horn/20 30, 15 10–80 PL, RMSDS,
PDP, AS
[107] LOS Spain WBTD 39/7.1 Horn, Omin/20.9, 3.5 14, 26 5–50 PL
[79] Both USA WB 60/10 OEW, horn/6.7, 29 90, 7 2.4–60 PL, RMSDS,
PAP
Table 4. MmWave channel measurement in laboratories.
Source Scenarios
LOS, NLOS Region Meas.Type Frq. (GHz)
/1
B(ns)
Antenna Type
/Gain (dBi)
AzHPBW (°)
/EvHPBW (°)
Distance
(m) Channel Statistics
[93]Both UAE NB 28/- Horn/15 - 1–60 PL
[96,97] Both USA NB 30 Omni - 1–6.5
8–13.5 PL
[108] – Hungary NB 38 Horn - 1–36 RSS
[100–102]Both Spain WBFD 30, 39, and
40/0.026 Omni - 0.5–13 PL, K-factor,
RMSDS
[109] – China NB 45 Horn/25 10 8.5 Diffraction
[110] Both France WBTD 60/2 Horn, patch/22.4, 3 12, 60/10, 60 8.7–13.2 Human effects
[29] Both Germany WBTD 60/1, 0.2 Omni - 0.5–10 PL, RMSDS
[111] Both Greece NB 60 Horn/21 36/11 0.5–15 PL, Fading statistics,
people effect
[104] Both Greece WBTD 60/8 Horn/21 36/11 - RMSDS, excess delay

Electronics 2021,10, 1653 11 of 28
Table 5. MmWave channel measurement in indoor libraries and other indoor environments.
Source Scenarios
LOS, NLOS Region Meas.
Type
Frq. (GHz)
/1
B(ns)
Antenna Type
/Gain (dBi)
AzHPBW (°)
/EvHPBW (°)
Distance
(m) Channel Statistics
[112] Library (Both) USA WBTD 28/0.5 Horn/17 24/26 10–50 PADP, PL,
RMSDS
[29] Library (Both) Germany WBTD 60/1, 0.2 Omni - 0.5–10 PL, RMSDS
[77] Lobby (LOS) China WBTD 39/0.5 Horn, Omni - 2–10 PL, RMSDS,
K-factor
[98] Courtyard USA NB 31 Horn/10 54/54 1–67,
18–25 PL
[113] Shopping mall (LOS) China WBTD 28/0.833 Horn/25 10/11 - Human
shadowing
[114] Shopping mall (Both) Finland WBFD 28/0.25 Horn, bicone
/19, 0 10/40, 60 <35 PL
[65,66]Shopping mall
railway station (LOS) Finland WBFD 61–65, 69–74
/0.25, 0.2 Omni, Horn/5, 20 20/20 0.9–10.3 Scattering
[93] Tunnel (Both) UAE NB 28/- Horn/15 - 1–60 PL
[115]Indoor airport
terminal (Both) USA NB 31 Horn/10 54/54 2–14 PL
[116,117]Indoor airport
terminal (Both)
South
Korea WBTD 2 Horn/10 and 24.4 /60 and 10 28–300 PL
The RMSDS, Angle of arrival (AOA), Angle of departure (AOD) and angular spread
(AS) were investigated in [
42
,
43
,
47
,
50
] at 28 GHz and in [
60
,
61
] at 60 and 70 GHz, based on
WBTD measurements using horn antennas at both Tx and Rx sides. The channel impulse
response (CIR), power angular profile (PAP), power angular spread (PAS), power angular
and delay profile (PADP) and K-factor channel statistics were presented in different indoor
office environments at various mmWave frequencies. Table 1summarizes a comparison of
mmWave channel measurements for indoor office environments.
4.2. Indoor Room, Hall, and Hallway/Corridor
The halls are characterized by large spaces and sometimes limited objects. The rooms
are characterized by smaller spaces and more objects. Corridors are usually long passages
in a building from which doors lead into rooms.
The study from [
91
,
92
] has shown that the size of the hall and position of the Tx and
Rx can affect the delay spread and the angle spread. Higher values of delay spread were
obtained in hall compared to the smaller environment like the classroom and office but
with lower angle spread. Wei et al. [
74
] compared the spatial-temporal characteristics of
28–30 GHz to other frequencies 2–4, 14–16 GHz in LOS and obstructed-LOS scenarios in
an empty room. The results show the 28–30 GHz has similar PADP with the 14–16 GHz,
however, a richer multipath environment was obtained at 2–4 GHz due to diffuse scattering
effects. Furthermore, few scatters were observed at 28–30 GHz in the empty room which
indicated the lack of frequency dependency.
A comparative study of measurement versus ray tracing simulation using PAP was
presented for large empty room and office scenarios [
51
]. The results of the empty room
scenario from the measurements and simulations were in agreement and characterized
by sparse delay and angle domains, with only a few dominant paths. In [
75
], a wideband
measurement for indoor environment with tables, computers and enclosed with glass
windows and a room was reported for 30 GHz. An RMSDS of 28 ns at 95% CDF value was
obtained for dual polarized antennas with the Tx positioned at 2.35 m AGL and the Rx at
1.6 m AGL in a near LOS scenario. In [
71
], directional and omni-directional large-scale path
loss models were investigated in a dining room for 28 GHz and compared with 38 GHz. It
was observed that the location of the Rx and the density of the surrounding objects in room
can affect the RMSDS and PL. Smaller RMSDSs were noticed in LOS boresight locations

Electronics 2021,10, 1653 12 of 28
compared to gypsum board and glass window locations due to less obstruction and multi-
path effects. In [
77
], the study for large scale and small scale propagation characteristics
for 39 GHz was carried out in a conference room and lobby. The results from the lobby
indicated rich MPCs, greater values of RMSDS and RMS AS when compared to the 3GPP
channel model. The summary of a comparison of mmWave channel measurements for
indoor rooms and hall environments is given in Table 2.
The experimental results in [
72
] showed that the propagation link between the corri-
dors and rooms was affected due to the penetration loss. For instance, a 12 dB loss was
observed in the 28 GHz compared to the 2 GHz in the same corridor. In [
35
,
36
], the CI
and FI path-loss models were investigated using the V-V and V-H antenna polarizations.
The PLEs for the V-V and V-H configurations obtained for the corridors were
less than 2
.
It was observed that the MPCs added up constructively due to the side walls along the
corridor. A comparative study of 28 GHz and 3.5 GHz was conducted in [
94
] and the
diffraction loss (DL) and frequency drop (FD) were investigated. It was observed that the
DL for the 28 GHz was twice that of the 3.5 GHz. The Path loss and small-scale fading
were investigated in [
96
] using directional antennas at 30 GHz in a LOS scenario on the
corridor. The strength of the direct path signal was used to determine the coefficients
K
and
m
that best fit the Rician and Nakagami distribution models, respectively. A Good fit
of the model was obtained from the computation of the measured data but the values
K
and
m
are affected by the distance between Tx and Rx. However, the Rician factor
K
was
computed by trial-and-error, while the Nakagami factor
m
was directly computed from
the measured data. The modified-CI model offered the same PLE as FI model with less
computational complexity. In [
103
], the propagation characteristic in a confined corridor
with a corner was investigated with highly directional antennas at 41 GHz. The result from
the study showed that the angle of the corridor corner can result in power loss.
The study from [
90
] investigated the use of two types of combinations of antennas
that is when Tx and Rx are both horn (horn-horn) antennas and when Tx is horn and
the Rx is omnidirectional (horn-omni) antenna using a channel-sounding with swept-
frequency method. Measurement campaigns were carried out in the corridor for 57 to
64 GHz frequency band. The result showed that limited AOA is observed in MPCs due
to the guided nature of propagation in the corridor without much difference in the horn-
horn and horn-omni antenna combination. Studies from [
105
] showed a relative lower
values of RMSDS for measurement campaigns at 60 GHz in different corridors. The
propagation effects and characteristic for 74 GHz in an entrance hall was reported in [
106
].
A comparison of mmWave channel measurements for indoor corridors and hallways
environment is summarized in Table 3.
4.3. Indoor Laboratory
The laboratory environment is composed of workbenches, desks, equipment, tables
and enclosed by walls and ceiling made of different materials, and windows. In [
93
], it was
reported that the PLE is identical with free space path loss exponent (FSPLE) based on NB
measurements at 28 GHz in indoor laboratory. In [
96
,
97
], statistical coefficient of Rician
and Nakagami distribution functions for 30 GHz propagation channel using measurement
results in a LOS showed that the value of the coefficient varies with the distance between
the Tx and Rx, and a good fit was obtained for small scale fading characteristics. The
measurement system was designed and developed in [
108
] for an indoor propagation map
at 38 GHz which showed that propagation conditions were largely affected by reflection. A
measurement campaign was presented in [
100
,
102
] for frequency between 1 and 40 GHz
for LOS and NLOS. The RMSDS, PL and K-Factor for various ranges of frequency were
presented in [
100
] while the reverberation time was shown to decrease with frequency
in [
102
]. The diffraction due to blockage from cylindrical blocks with different materials
and the human body at 45 GHz was presented in [
109
]. The signal attenuation by the
human body was found to be smaller than the cylindrical block which indicated that the
diffraction by the human body was better.

Electronics 2021,10, 1653 13 of 28
Multipath parameters for wideband 60 GHz channel were reported in [
104
]. The
RMSDS values of 12 ns in LOS and 21 ns in NLOS scenario were observed and the channel
also exhibited enhanced frequency selective characteristics. The temporal variation of the
60 GHz channel due to the presence of humans was reported in [
110
,
111
]. The propagation
characteristic for 60 GHz for environment with and without the movement of people in a
LOS and NLOS was presented [
111
]. The number of bodies, speed and the propagation
environment resulted in a fluctuation of the received signal between fixed terminals. More
studies on the effect of human activity from the number of people between zero and fifteen
persons for a 60 GHz channel were reported in [
110
]. The results showed that direct part
shadowing from the human body can result in attenuation of more than 20 dBm for some
duration based on the number of people. Table 4summarizes a comparison of mmWave
channel measurements for indoor laboratory environments.
4.4. Other Indoor Scenarios
Rising interest to use the mmWave in different application scenarios led to indoor
channel measurements in the indoor library, indoor lobby and courtyard, indoor shopping
malls, indoor hospital, indoor tunnels, indoor airport terminal building, maintenance
hangar, and hall in passenger terminals, and indoor railway station.
A Study in a shopping mall was conducted in [
114
] using a wideband directional
channel measurement. A high correlation was observed in the multipath components of
the 28 and 140 GHz. Propagation studies in large halls inside the airport from [
116
,
117
]
reported PLEs in LOS close to that of free space curve (n = 2) where the Tx and Rx were
positioned at 8 m and 1.5 m, respectively. Table 5gives a comparison of mmWave channel
measurements for indoor libraries and other indoor environments.
Some measurements were conducted at W-band (75–110 GHz) for a short distance in
indoor environments. In [
118
,
119
], the measurements were conducted at W-band for a very
short distance in an indoor environment to study the reflection from wood, polymer, and
metal [
119
] and the channel variation based on PDP at 5–25 cm Tx-Rx separation distance
with 5 cm step [
118
]. It was found that the PLE of W-band is almost identical with free
space PLE.
5. Path Loss and RMSDS Results and Discussion
5.1. Path Loss Analysis
Based on all mentioned path loss models in Section 3.1, the path loss mainly depends
on the operating frequency, Tx-Rx separation distance, the environment effects (presented
in the PLE (
n
) and shadowing effects (
Xσ
). Table 6presents the CI and FI path loss model
parameters based on different measurements in indoor office environments for LOS and
NLOS scenarios among different mmWave frequencies. The CI and FI path loss model
parameters for different indoor halls, corridors, libraries, and laboratories are presented
in Table 7. Based on the CI path loss model, Table 6shows that the PLE values for LOS
indoor office environments are equal to or less than of FSPLE value of 2 for all studied
mmWave frequencies. In some indoor office environments, the PLE (
n
) is lower
(below 1.5)
and even below 1. This implies that there are strong MPCs from different scattering objects
and many MPCs are added up constructively. In [
37
], the PLE values are up to 4.7 for
28 and 73 GHz in the indoor office. These values are abnormal as compared with other
studies. The high PLE which is more than FSPLE in LOS indoor environment is due to the
cross-polarization between the Tx and Rx antenna. In [
37
], the results showed that using the
same environment with the same measurement setup, the PLE values are 1.1 and 1.3 at 28
and 73 GHz, respectively, using V-V polarization. However, for the V-H (cross) polarization,
the PLE values are 2.5 and 3.5 at 28 and 73 GHz, respectively. This implies that the received
power degraded by 14 and 22 dB/decade using cross polarizations at 28 and 73 GHz,
respectively. The 3GPP FI path loss model for LOS indoor office environment shows the
line slope
β
values are less than the FSPLE value for all listed mmWave frequencies in
Table 6.

Electronics 2021,10, 1653 14 of 28
The PLE and
β
values are less than the FSPLE value for all frequencies in LOS indoor
corridors [
35
,
36
,
46
,
72
,
76
,
91
–
95
,
98
–
100
] as depicted in Table 7. The PLE values are around
FSPLE value in large halls as in [
107
,
116
,
117
]. In [
116
,
117
], the PLE for LOS at 28 GHz
along 300 m big hall in passengers terminals is 2.2 (more than the FSPLE value of 2 by 0.2).
In [
107
], the PLE along 50 m LOS hallway is 2 at 39 GHz. For the library [
112
] and the
laboratory [107] environments, the PLE values are identical with FSPLE value.
Table 6. Path Loss and Delay Spread for different studies in indoor offices.
Source Freq. (GHz) Scenarios CI Model
n,σ(dB)
FI Model
β,σ(dB) RMSDS (ns)
[37] 28 LOS
NLOS
1.1–4.1, 1.8–8.0
2.7–5.1, 9.4–11.6
0.8–1.4, 1.4–2.0
2.3–3.6, 9.3–10.6
0.7–134.4
0.6–198.5
[43] 28 LOS
NLOS
1.9, 2.1
2.8, 6.2
1.7, 2.1
1.5, 5.8
-
<100 (18.9)
[44] 28 LOS
NLOS -1.9, 2.2
3.6, 2.9
20–50
20–50
[72] 28 NLOS - 2.3, 3.4 -
[46] 28 LOS
NLOS
1.6 and 1.8, 0.7 and 0.6
2.1 and 2.5, 2 and 3.5
1.3 and 1.5, 0.9 and 0.8
1.2 and 1.4, 1.6 and 3.2 -
[47] 28 LOS
NLOS
1.5, 1.7
2.2, 3.4
1.4, 1.7
2.2, 3.3
5–20
10–40
[49] 28 LOS 1.6–1.8, 0.7–2.7 - 5–25
[50] 28 LOS 1.7, 1.3 and 3.1 - 10–30
[75] 30 LOS - - 2.5–28
[52] 30 LOS 2.0, 0.1 1.9, 0.1 –
[55] 32 and 39 LOS 1.9 and 1.8, 2.5 and 2.2 - 24.3 and 20.5
[54] 38 LOS
NLOS
2.0, 4.6
2.9, 6.8
0.9, 3.1
1.3, 6.2 –
[120] 37.2 LOS 1.5–2.1, 4.4–5.5 - 8–16
[80] 60 LOS - 1.3, - -
[29] 60 LOS - 1.3, 5.1 5–20
[83] 60 LOS 1.9, 1.7 -
[84] 60 LOS
NLOS
0.6–1.2, 1.3–2.7
2.7–5.4, 2.7–3.9
-
--
[58,59] 60 LOS 2.1, 7.9 - 2–30
[60] 60 NLOS - - 3.3–5.7
[87] 58 LOS
NLOS
1.2, 2.7
5.4, 3.9
-
-
5–35
5–45
[66] 60 LOS - - 5–20
[69,70] 60 LOS 1.6–1.8, 0.1–1.1 - -
[62] 70 LOS
NLOS
1.7, 2.2
2.9, 6.8 - -
[37] 73 LOS
NLOS
1.3–4.7, 2.4–8.6
3.2–6.4, 9.7–15.9
0.5–1.6, 1.4–4.6
1.3–2.7, 7.5–11.3
0.5–143.8
0.5–142

Electronics 2021,10, 1653 15 of 28
Table 7. Path Loss and Delay Spread for different studies in hall, corridor, library, and laboratory.
Source Freq. (GHz) Scenarios CI Model
n,σ(dB)
FI Model
β,σ(dB) RMSDS (ns)
[35,36,94] 28 LOS
NLOS
0.6–1.8, 2.1–3.8
3.6, 5.7
0.9–1.2, 2.0–3.1
3.0, 5.3
1–11.7
1–11
[116,117] 28 LOS
NLOS
2.2, 1.2 and 1.3
3.0 and 2.7, 7.8 and 5.3 - -
[72] 28 LOS - 1.7, /2.4 -
[46] 28 LOS 1.3 and 1.5, 0.9 and 0.8 1 and 1.3, 1.0 and 1.3 -
[112] 28 LOS
NLOS
2.1, 4.2
3.3, 13.5
1.8, 4.1
1.3, 13.0
27.5–45
25–63
[91,92] 28 LOS - 1.5, 0.5 5–25
[95] 28 LOS
NLOS
1.9, 4.2
--<60
<60
[93] 28 LOS 1.7–2.1, 0.7–5.5 - -
[98] 31 LOS
NLOS
1.7, 2.2
2.9, 6.8
-
--
[35,36] 38 LOS 0.8–1.3, 1.3–3.3 0.9–1.5, 2.3–4.3 1–11
[76] 40 LOS 1.8, 4.7 1.8, 4.7 -
[100] 30 and 40 LOS - 1.4, - 1–8
[101] 30 and 39 LOS
NLOS
-
-
1.9 and 1.8, -
2.8 and 2.4, -
2–10
1–8
[107] 39 LOS 2 and 1.5, 5.1 and 4.1 2.1 and 1.3, 5.1 and 4.0 -
[103] 41 NLOS 1.6–2.2, 2.2–3.0 - 1–10
[79] 60 LOS 1.9, 8.6 - 4.6–47.3
[111] 60 LOS
NLOS
1.8, 1.1
2.0, 3.8
-
--
[104] 60 LOS
NLOS
-
-
-
-
12.3–21.1
18.5–31.7
[105] 60 LOS - - 5–30
[99] 30 and 60 LOS
OLOS
NLOS
1.7 and 1.8, 3.7 and 3.8
1.8 and 1.9, 1.2 and 1.4
2.3 and 2.5, 5.9 and 7.2
-
-
-
6.7 and 3.4
27.1 and 23.5
28.3 and 22.5
[90] 60 LOS - - <70
The standard deviation values of the shadowing effect (deviation from log-linear-fit)
vary from 0.1 to 5.5 dB for CI model and between 0.1 and 5.1 dB for FI model at all listed
LOS studies in Tables 6and 7. The large standard deviation values of 8, 8.6 dB at 28
and 73 GHz, respectively, for CI models [
37
], are for cross polarizations and 8.6 dB at
60 GHz [79] is due to the penetration effects for one measurement locations.
The PLE values for NLOS indoor office environments vary from 2.1 to 5.4 for all listed
frequencies in Table 6. In [
37
], the PLE for cross polarization at 73 GHz is 6.4. The
β
of
the 3GPP FI path loss model in NLOS indoor office vary from 1.3 to 3.6
[37,43,44,46,54]
for different mmWave frequencies. The PLE values for NLOS indoor corridor environ-
ments
[94,98,103,111,116,117]
vary from 1.6 to 3.6 at different frequencies as shown in
Table 7
. The PLEs for NLOS scenarios vary from 2 to 3.3 for hall [
116
,
117
], library [
112
],
and laboratory [
111
]. The
β
values for the FI model are 3 and 1.3 at 28 GHz in the indoor cor-
ridor [
94
] and library [
112
]. It is worth mentioning that the PLE is not frequency-dependent

Electronics 2021,10, 1653 16 of 28
in an indoor environment and it depends on the structure of the environment and the type
of environment.
It can be noted from the PLE and
β
values of the CI and FI path loss models are similar
for LOS scenarios for most of the listed studies in Tables 6and 7, however, for NLOS
scenarios, there is a deviation between both models in most of the indoor environments.
Figure 5presents the CI and FI path loss models along 100 m at 28 and 38 GHz for
LOS and NLOS scenarios. It can be shown that the PLEs at 28 GHz are 1.9 and 2.8 in indoor
offices for LOS and NLOS, respectively, [
43
]. In the same environment, the slope line values
for the FI model are 1.7 and 1.5 for LOS and NLOS, respectively. The CI and FI models
for LOS are comparable (0.2 difference in PLEs, which is around 2 dB per decade). The
deviation of the FI model comparing with CI based on the reference distance (1 m in most
cases of the indoor environments) can be explored from the deviation of floating-intercept
from free space path loss at the reference distance. In [
43
], the
α
value of the FI model is
63.2 which is more than the FSPL value at
d0
of 1 m by 1.8 dB. However, for the NLOS
scenario, the
α
value of the FI model is more than the FSPL value at
d0
of 1 m by 18.7 dB
from FSPL at a reference distance of 1 m.
In [
54
], the same findings were explored for 38 GHz in the indoor office environment
as shown in Figure 5. It can be seen that for the LOS scenario, the PLE for the CI model
and
β
for FI model are 2 and 0.9, and the
α
value of the FI model is more than the FSPL
value at
d0
of 1 m by 14.3 dB. The NLOS line slope of FI model is less than the PLE of the
CI model by around 16 dB per decade. The αvalue of the FI model is more than the FSPL
value at
d0
of 1 m by 19.6 dB. Base on all mmWave studies in different LOS and NLOS
indoor environments, the FI and CI path loss models can fit the experimental data well in
LOS scenarios, however, for NLOS scenarios the CI model is preferred.
The path loss is calculated for LOS and NLOS indoor environments based on the CI
model for different mmWave bands; namely 30 GHz, 40 GHz, and 70 GHz as shown in
Figures 6–11
. As it is noted from Tables 6and 7, for all listed studies, the PLE trend is not
monotonic with frequency, hence, the same PLE can be applied for different frequencies. For
each band, Figures 6–11 show that with the same PLE, the path loss values are comparable
for all listed frequencies in the same band, i.e., at 30 GHz band (28 GHz, 30 GHz, 32 GHz,
and 39 GHz), using 0.6 PLE the maximum path loss difference between the lowest frequency
in the band 28 GHz and the highest one 39 GHz is around 3 dB. The path loss is affected
by the construction and the structure of the environments, which can be explored from
PLE values. For example, in a narrow corridor and closed indoor office with concrete walls
available (acts as a wave-guide for the received signal), the PLE is less than 1 for different
frequencies [35,36,84,94].
5.2. RMSDS Analysis
The RMSDS values vary from 0.5 to 134.4 ns and from 0.5 to 198.5 ns for LOS and
NLOS scenarios, respectively, among all listed frequencies in Tables 6and 7. Based on
these studies in different indoor environments, the RMSDS trend is not monotonic with
Tx-Rx separation distance and frequency. It depends on the structure of the environment
and the measurement setup. For the narrow beamwidth of the antenna and the small
indoor environment, the RMSDS is small. The delay spread of mmWave frequencies is
considerably similar to the currently used frequency bands below 3 GHz. Tables 6and 7
presents the RMSDS for most studies in the literature at mmWave frequencies.

Electronics 2021,10, 1653 17 of 28
Figure 5. Comparision of CI and FI models at 28 and 38 GHz for LOS and NLOS environments.
Figure 6. Path Loss at LOS environments for 30 GHz bands.

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Figure 7. Path Loss at LOS environments for 40 GHz bands.
Figure 8. Path Loss at different LOS environments for 70 GHz bands.

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Figure 9. Path Loss at NLOS environments for 30 GHz bands.
Figure 10. Path Loss at NLOS environments for 40 GHz bands.

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Figure 11. Path Loss at NLOS environments for 70 GHz bands.
5.3. Summary
Based on the review of many results of the path loss model of mmWave bands at
different frequencies from 28 to 73 GHz in different indoor environments at different
regions in the world, the operating frequencies for indoor environments can be grouped
into three different bands named 30 GHz, 40 GHz, and 70 GHz. The 30 GHz band represents
the frequency range from 28 to 39 GHz while the 40 GHz band and 70 GHz bands represent
the frequency range of 40–45 GHz and 58–73 GHz, respectively. From this study, it can be
observed that for the indoor environment with the same physical characteristics the change
of the carrier frequency within the same band does not contribute much. For example,
using the CI path loss model with PLE of 1.6 and
d0
of 1 m the path loss values at such
particular Tx-Rx separation distance, i.e., 100 m for 28 GHz (lowest frequency at 30 GHz
band) and 39 GHz (highest frequency at 30 GHz band) are 64.5 dB and 67.5 dB, respectively.
This implies that the path loss value at 39 GHz is only increased by 3 dB compared to the
value of path loss at 28 GHz.
From different LOS and NLOS indoor environments studies at mmWave bands,
the FI and CI path loss models can fit the experimental data well in the LOS scenario,
however, for the NLOS scenario, the CI model is preferred. Based on these studies in
different indoor environments, the RMSDS trend is not monotonic with Tx-Rx separation
distance and frequency. It depends on the physical structure of the environments and the
measurement setup.
6. Future Trends
From the survey of the existing works, we discuss the future trends based on the
following areas.
6.1. Critical Indoor Environment
The 5G wireless networks will be used for different indoor applications such as emer-
gency cases. A common part of any building structure is the stairs, essentially used in emer-
gency cases for fire escape or in natural disasters. Few propagation measurements have
been conducted at mmWave for indoor channels in the stairwell [
121
–
123
]. In
[121,122]
, the

Electronics 2021,10, 1653 21 of 28
path loss was investigated using CI, FI, ABG, and CIF models based on NB measurements
at 28 GHz, 32 GHz, and 38 GHz that conducted in two different stairwells in a tropical
region, i.e., Malaysia. The path loss models in the stairwell environments indicated that
the received power decay is high where the PLE is more than 7. More studies are needed
to investigate the radio propagation channel at mmWave in stairwell environments.
Furthermore, propagation measurement at mmWave band in indoor industrial envi-
ronments is expected to attract more research interest [
124
]. For instance, the mmWave
channel properties were investigated at 28 and 60 GHz based on wideband measurements
using VNA in two different factories, which represent light and heavy industry [
125
]. The
wireless channels need more characterization in different indoor industrial environments.
6.2. Artificial Intelligence
The use of artificial intelligence (AI) methods such as machine learning and deep
learning are now been applied in the field of wireless communication. Recent studies have
investigated the use of machine learning for the prediction of path loss models [
126
–
129
].
The result of these studies has shown a positive reduction in complexity, amount of time
required in measurement and an enhancement in path loss models. Further research is
expected in the application of machine learning for the prediction of channel models for
indoor environments. Moreover, deep learning is also used to model the path loss for 5G
communications with static objects [
130
]. The influence of moving objects such as vehicles
and the heights of Tx and Rx antennas are still an open issue which can be addressed for
path loss modelling using deep learning.
6.3. Channel Characterization for Indoor Devices
The use of mmWave for dense indoor devices is expected to solve the problem of
bandwidth scarcity needed for high data rates. In addition, the mmWave can provide
narrow beam communication for device-to-device communication in indoor environments.
Examples of devices that benefit from the application of mmWave are wearable devices as
presented in [
131
,
132
]. The need for characterization of propagation studies for different
devices operating at mmWave for indoor environments is expected to attract further
research interest.
6.4. Reconfigurable Intelligent Surface
The use of reconfigurable intelligent surfaces (RIS) is considered a promising network
architecture for 5G, 6G and beyond. The RIS makes use of a surface composed of
N
elements
that are reconfigurable to collect wireless signals from a Tx and passively beamform them
towards the desired Rx [
133
,
134
]. This is expected to help improve the propagation in
indoor environments and overcome the limitations of signal loss attributed to mmWave
propagation. Hence, more investigation of path loss models is needed in mmWave for RIS
as presented in the recent studies [
135
–
137
]. In addition, it is found that most researchers
used simplified models for the wireless channel (i.e., i.i.d Rayleigh fading channel). The
channel fading is applicable for inexpensive antenna whose inter-distance is larger than half
of the wavelength, but their application to metasurface-based RISs made with more fading
factors such as small-scale-fading necessitates further investigation [137,138]. In addition,
the uses of models that accounts of spatially-stationary scattering channel such as the plane-
wave model (isotropic) and plane-wave model (non-isotropic) for metasurface/antenna
arrays proposed in [139,140] needs to be explored for mmWave indoor environment.
6.5. MmWave for 6G
The current candidate 5G frequency band (24 to 86 GHz) may not be sufficient due to
the fast emerging wireless data traffic and emerging applications. Hence, the 6G wireless
system is focused on exploring above 100 GHz to overcome spectrum scarcity and band-
width limitation [
141
]. Furthermore, more communication is envisaged to be done indoors
since 80% of the time people stay in indoors [
142
]. Hence, more research work is expected

Electronics 2021,10, 1653 22 of 28
in the study of mmWave frequencies from 100 to 300 GHz and beyond (sub-THz and THz
wireless networks) in indoor environments [
143
]. This is expected to open several research
issues on propagation models at various indoor locations, and penetration measurement
for different materials. The performance of the current 5G mmWave frequency bands can be
compared with 6G proposed frequency bands. Furthermore, the 6G channel measurement
is time consuming, and the design of a high-performance channel sounder that can fulfill
the requirement of 6G channel is still open research [144].
6.6. Near-Ground Propagation Measurements
The propagation measurement of mmWave near-groud is still a challenging and
interesting research area. The mmWave band will be used in V2V communication in which
two antennas in the front and rear bumper of vehicles are installed. In such cases, the Tx
and Rx antennas are located close to the ground [
145
,
146
]. Furthermore, The internet of
things (IoT) and sensors application operating on mmWave and propagate near the ground
will open a new direction for future research [147].
7. Conclusions
A review of indoor mmWave channel measurements at different candidate frequencies
for 5G wireless networks has been presented. The indoor channel characteristics of the
mmWave band from two main wireless channel perspectives, that is, the path loss and
time dispersion have been discussed. While the path loss increases as the operating
frequency increases, the path loss exponent is not frequency-dependent, but it depends
on the environment type and structure. The best models for path loss in the indoor
environment at the mmWave bands such as the CI and FI models for LOS channels and
CI model for NLOS channels were identified. It was noted that the mmWave indoor
channel has a small RMSDS value for narrow HBPW and a small size indoor environment.
The future trends for the mmWave indoor environment have been discussed in this paper.
Further research works are needed in the study of mmWave industrial indoor environments,
indoor stairwell environment, propagation studies for wearable devices, reconfigurable
intelligent surfaces, and application of artificial intelligence. This survey has provided a
comprehensive review of the work that has been conducted on measurement campaigns
and propagation models for the indoor mmWave environment. The engineering and
research community will find it helpful in the design of an optimum indoor wireless
network, and the performance of mmWave for 5G, 6G wireless networks and beyond.
Author Contributions:
Conceptualization, A.A.-S.; Data curation, A.A.-S.; Formal analysis,
A.A.-S.
,
M.C. and O.E.; Funding acquisition, A.A.-S. and M.C.; Investigation, A.A.-S., M.C. and O.E.; Method-
ology, A.A.-S., M.C. and O.E.; Project administration, A.A.-S. and M.C.; Resources, A.A.-S. and O.E.;
Software, A.A.-S. and O.E.; Supervision, A.A.-S., M.C. and S.K.A.R.; Validation, A.A.-S., M.C., O.E.
and S.K.A.R.; Visualization, A.A.-S., M.C. and O.E.; writing—original draft preparation, A.A.-S.,
M.C. and O.E.; Writing—review and editing, A.A.-S., M.C., O.E., S.K.A.R., Y.A.A.-G. and T.A.-H. All
authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by Manu Lab, NTNU, Gjøvik.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.

Electronics 2021,10, 1653 23 of 28
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