mmWave Paper

Millimeter Wave
Challenges and Solutions
Submitted to:
Dr. Cheng Li
Written by:
Lee Stewart
009414657
12/7/2015

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Table of Contents
Introduction..................................................................................................................................... 1
Millimeter Wave Spectrum............................................................................................................. 2
Millimeter Wave Applications........................................................................................................ 2
Millimeter Wave Channel Characteristics...................................................................................... 2
28 GHz........................................................................................................................................ 3
Penetration and Reflection Analysis....................................................................................... 4
Path Loss and Signal Outage Analysis ................................................................................... 6
Angle-of-Arrival (AOA) and Angle-of-Departure (AOD) Analysis...................................... 8
38 GHz...................................................................................................................................... 10
60 GHz...................................................................................................................................... 12
73 GHz...................................................................................................................................... 13
Millimeter Wave Challenges ........................................................................................................ 14
Integrated Circuits and System Design..................................................................................... 14
Interference Management and Spatial Reuse............................................................................ 15
Anti-Blockage............................................................................Error! Bookmark not defined.
Dynamics Due to User Mobility............................................................................................... 16
Millimeter Wave Solutions........................................................................................................... 17
Wireless Backhaul .................................................................................................................... 17
Integrated Circuits and System Design..................................................................................... 19
Interference Management and Spatial Reuse............................................................................ 20
Anti-Blockage........................................................................................................................... 22
Dynamics Due to User Mobility............................................................................................... 23
Conclusion .................................................................................................................................... 24
References..................................................................................................................................... 26

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Table of Figures
Table 1: 28 GHz Penetration losses using 24.5 dBi horn Antenna with 10 degree half power
beamwidth....................................................................................................................................... 5
Table 2: 28 GHz propagation losses through multiple indoor obstacles ........................................ 5
Table 3: 60 GHz parameters for path loss model ......................................................................... 12
Table 4: 28 and 73 GHz PLEs and standard deviations for directional and omnidirectional PL
models and for co-polarization ..................................................................................................... 13
Table 5: 28 and 73 GHz PLEs and standard deviations for both directional and omnidirectional
path loss models and for cross-polarization V-H scenarios.......................................................... 14
Table 6: 28 and 73 GHz mean RMS delay spread (ns) for co- and cross-polarization
combinations in LOS and NLOS scenarios .................................................................................. 14
Table 7: Channel capacities under different distances.................................................................. 16
Table 8:MGB wireless backhaul link budget................................................................................ 18
Figure 1:28 GHz Tx Block Diagram .............................................................................................. 3
Figure 2: 28 GHz RX Block Diagram ............................................................................................ 4
Figure 3: 28 GHz reflection coefficients using horn antennas with 24.5 dBi gain and 10 degree
half power beamwidth..................................................................................................................... 6
Figure 4: 28 GHz sectorized map ................................................................................................... 7
Figure 5: 28GHz maximum coverage distance with 119 dB maximum PL................................... 8
Figure 6: Polar plot showing the received power at NLOS locations............................................. 9
Figure 7: Power delay profile measured over 10-wavelength linear rack at 28 GHz................... 10
Figure 8: RMS delay spread as a function of arc lenth at 38 GHz ............................................... 11
Figure 9: MGB Backhaul.............................................................................................................. 17

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Introduction
The current cellular spectrum is between 700 MHz and 2.6 GHz which is spread between
2G, 3G, and 4G systems (Rappaport et al., 2013, p. 335). However, the exponential growth in
data traffic as a result of the proliferation of smartphones will constrain these systems. Moreover,
the future will see more connected devices (such as wearable devices and vehicle-to-vehicle
(V2V) communication) and add-on services (such as ultra-high definition video and 360° video)
which will further lead to bottlenecks in the wireless network (Pi, Choi, & Heath, 2015, p. 1).
This had led to a lot of research into the next generation communication system, 5G which is
anticipated around 2020 (Rappaport et al., 2013, p. 336).
Although there are no definitive standards developed, some of the goals of 5G are
thousand-fold system capacity, hundred-fold energy efficiency, and tens of lower latency (Gao,
Dai, Mi, Wang, Imran & Shakir, 2015, p. 1). Boccardi, Heath, Lozano, Marzetta, & Popovski
(2014) identified five technologies that will play a role in 5G. They are device-centric
architectures, millimeter wave (mmWave), massive MIMO (multiple input multiple output),
smarter devices, and native support for machine-to-machine (M2M) communication. Device-
centric architecture involves changes to uplink, downlink, control channels, and data channels to
better route information. mmWave involves using higher frequency waves where there is more
spectrum available. Massive MIMO involves using a lot of antennas (up to 100) to multiplex
signals, electronically direct beams, and reduce intra- and inter-cell interference. Smarter devices
involve sharing control between base stations and smartphones by allowing device-to-device
connectivity and exploiting smart caching at the device side. Native support for M2M
communication involves providing low-rate-data services to support a massive number of low-
rate devices, providing a minimum data rate, and ensuring very-low-latency data transfer (p. 1-
2).
This research paper will delve into the mmWave component of 5G. Topics to be
discussed will include mmWave spectrum, mmWave applications, mmWave channel
characteristics, mmWave challenges, and associated mmWave solutions.

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Millimeter Wave Spectrum
The mmWave spectrum spans from 30 to 300 GHz which maps to 10 mm to 1 mm
wavelength respectively (Athanasion, Chathuranga, Fischione, & Tassiulas, 2013, p. 1).
However, only a fraction of this spectrum is of interest. These include the following:
 Local multipoint distribution service (LMDS) band (27.5 – 28.35, 29.1 – 29.25, and 31.0
– 31.3 GHz)
 39 GHz band (38.6 – 40.0 GHz) (Pi et al., 2015, p. 1)
 V-band (57- 67GHz) (Gao et al., 2015, p. 2)
 E-band (71-76 GHz, 81-86 GHz and 92-95 GHz) (Boccardi et al., 2014, p. 3)
Millimeter Wave Applications
Two applications for mmWave are small-cell access and wireless backhaul. mmWave is
ideal for small-cells smaller than 200 m (to be discussed in the next section). Candidate bands for
this application are LMDS (28 GHz), 38 GHz, and E-band (71-76 and 81-86 GHz). Some of the
research indicates that mmWave will be able to provide data rates of approximately 10 Gbps
within the cell and 100 Mbps at the cell edge. One researcher proposed a system that supported
uncompressed high-definition (HD) video up to 3 Gbps. (Niu, Li, Jin, Su, & Vasilakos, 2015, p.
9).
mmWave can also be used for wireless backhaul between macro and small-cell base
stations. Currently, the only way to get gigabit backhaul is with fibre optic cables. However,
mmWave has the advantages of being more cost-effective than fibre, being more flexible, and
being easier to deploy. (Niu et al., 2015, p. 10). The advantages of mmWave backhaul are:
 It has gigahertz bandwidth
 A large number of antennas can be used because of the small wavelength which can
improve signal directivity and link reliability and ensure a small form factor
 The high path loss (discussed in next section) reduces inter-cell interference and
improves frequency reuse (Gao et al., 2015, p. 1-2).
Millimeter Wave Channel Characteristics
This section will summarize the results of experiments conducted to study the
characteristics of the 28, 38, 60, and 73 GHz frequencies. Some of these characteristics include

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path loss (PL), penetration and reflection, angle of arrival (AOA), and angle of departure (AOD).
In the 28 and 38 GHz sections, the path loss equation is
𝑃𝐿 = 16𝜋2
(
𝑅
𝜆
)
𝑛
Where R is the range between transmitter (TX) and receiver (RX) and 𝜆 is the wavelength. Both
are in meters. n is the path loss exponent (PLE) (Swindlehurst, Ayanoglu, Heydari, & Capolino,
2014, p. 2).
28 GHz
The experiments using the 28 GHz frequency were conducted by Rappaport and his team
in 2012 in New York City which can be considered a dense urban environment. The hardware
they used consisted of a 400 Mcps sliding correlator channel sounder with 2.3 ns multipath
resolution. The TX and receiver RX block diagrams are below.
Figure 1:28 GHz Tx Block Diagram

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Figure 2: 28 GHz RX Block Diagram
A pseudo-random noise (PN) sequence sliding correlator was used to generate the
probing signal. The signal was modulated to a 5.4 GHz intermediate frequency (IF) and up-
converted to 28 GHz by mixing it with a 22.6 GHz local oscillator (LO). The transmitter power
was +30 dBm. This was fed to either a steerable 10° beam width 24.5 dBi horn antenna or a
manually rotated 30° beam width 15 dBi horn antenna. The TX used the same type of horn
antennas. The dynamic range between the TX and RX was 178 dB in order to obtain a SNR of
10 dB which is expected to be the SNR specification on future small cells (Rappaport et al.,
2013, p. 339-40).
Penetration and Reflection Analysis
The first experiment consisted of penetration and reflection measurements on different
material such as tinted glass, clear glass, brick, and drywall. The experiments were done indoors
and outdoors. The results for penetration losses are shown in the table below.

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T
Table 1: 28 GHz Penetration losses using 24.5 dBi horn Antenna with 10 degree half power beam width
From the table above it can readily be seen that tinted glass has high penetration losses of
40.1 dB (outdoors) and 24.5 dB (indoors). Brick also has a high loss of 28.3 dB (outdoors). On
the other hand, clear glass and drywall have low penetration losses. Clear glass has less than 4
dB loss and drywall has only 6.8 dB loss. The implication of the high path loss between the
internal and external environments is that there needs to be a bridge between both environments.
This could involve installing a repeater or an access point.
In addition, penetration measurements were also done at varying distances between TX
and several RXs indoors through multiple obstacles. The results are contained in the table below
and are relative to a 5 m free space test (i.e. TX and RX free from obstruction). As expected,
losses generally increased the further the TX and RX are from each and as more obstacles that
were in the way. Of course, the type of object matters as well. For example, RX 3 and RX5 had
the same loss of 45.1 dB even though RX3 was closer to the TX and had fewer obstructions. This
means that doors have a bigger effect on path loss than cubicles and walls. Also note that a
penetration loss of 64 to 74 dB results in a weak signal. A loss of more than 74 dB results in no
signal being detected.
Table 2: 28 GHz propagation losses through multiple indoor obstacles
The table below shows the reflection coefficients for tinted glass, clear glass, concrete,
and drywall. Similar to what was seen with penetration loss; tinted glass and concrete have high

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reflection coefficients of 0.896 and 0.815 respectively. However, concrete has relatively low
reflectivity of 0.623 at 45° angle. Likewise, clear glass and drywall have lower reflection
coefficients of 0.740 and 0.704, respectively (Rappaport et al., 2013, p. 339-342).
Figure 3: 28 GHz reflection coefficients using horn antennas with 24.5 dBi gain and 10 degree half power beam width
Moreover, rain and atmospheric absorption loses are negligible at 200 m from the base
station. For a heavy rainfall of 25 mm/h the attenuation is only 1.4 dB over this distance.
Likewise, atmospheric absorption is only 0.012 dB over 200 m (Rappaport et al., 2013, p. 338).
Path Loss and Signal Outage Analysis
This study employed the same types of TX and RX discussed in the previous section. The
only difference is that this experiment was exclusively outdoors. At the NYU-Poly campus in
Brooklyn, one TX and 11 RXs were used. The distance between the TX and RX ranged from 75
m to 125 m. At the NYU campus in Manhattan, 3 TXs and 75 RXs were used. The distance
between TX and RX ranged from 19 m to 425 m. At three of the locations in Brooklyn, the RX
was moved on an automated linear track of 10 wavelengths (107 mm) in half-wavelength
(5.35mm) increments to study small scale fading. At each track position, a 360 degree azimuthal
sweep was performed in steps of 10° (using the 10° beam width 24.5 dBi horn antenna) or 30°
(using the 30° beam width 15 dBi horn antenna). Large scale propagation characteristics were
investigated in the remaining eight RX locations in Brooklyn and all the Manhattan locations
using 24.5 dBi antennas. At each TX and RX location, measurements were taken for three
different TX azimuth angles, -5°, 0°, and +5° degrees from bore sight to the receiver and for
three different RX elevation angles of +20°, 0°, and +20. For each of the nine antenna pointing
combinations, the RX antenna was swept 360° in the azimuth plane in 10° steps and
measurements were recorded if energy was received. Each location had the potential to collect
324 power delay profiles (PDPs) for all combinations (36 RX azimuth angles, 3 TX azimuth
angles, and 3 RX elevation angles) (Rappaport et al., 2013, p. 342-3).
The PDPs contained numerous multipath with large excess delay for both line-of sight
(LOS) and non-line-of-sight (NLOS). The average number of LOS multipath components was

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7.2 with a standard deviation of 2.2 for a TX-RX separation of less than 200 m. The average
number of NLOS multipath components was 6.8 with a standard deviation of 2.2 for a TX-RX
separation less than 100 m. With a TX-RX 52 m separation a LOS measurement showed a
relatively large 753.5 ns excess delay. In a NLOS distance of about 423 m, the excess delay was
1388.4 ns. These results indicate that a signal can travel over a long distance through a highly
reflective environment to create a link. When path loss (PL) was calculated for all locations, the
best LOS path loss exponent (PLE) was 1.68. The average LOS PLE was 2.55. The average
NLOS PLE was 5.76. However, the average NLOS PLE was significantly reduced when the TX
and RX antennas where pointed at each other at the best possible angle. This resulted in an
average NLOS PLE of 4.58, which is very similar to NLOS path loss experienced in the 700
MHz - 2.6 GHz bands.
An outage study was also conducted to find the locations and distances where energy
could not be detected. The map below is sectioned into sectors corresponding to TX locations.
The yellow stars represent TXs, green circles represent acquired signals, red triangles represent
detected signals, and black X represents no signal detected. Signals were acquired by the RX
from all TXs within 200 meters. While most of the RX locations within 200 meters detected a
signal, in three cases the signal-to-noise ratio (SNR) was not strong enough for a signal to be
received. Of the measurements taken, 57% of outages were the result of obstructions. Most
outages occurred beyond 200 m from the TX.
Figure 4: 28 GHz sectorized map

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Factors affecting the outage probability include transmitted power, antenna gains, and the
environment. The figure below shows the relationship between the maximum coverage distance
of the base station and the combined TX-RX antenna gain. To calculate the maximum coverage
distance, the 49 dBi combined antenna gain was subtracted from the total PL of 178 dB. Since
the system requires a minimum of 10 dB SNR for a reliable signal level, the actual maximum
measureable path loss is 119 dB without including antenna gains. This was used to compute the
coverage distances corresponding to various antenna gains. The four blue curves show the cases
for PLEs equal to 3, 4, 5 and 5.76. The red squares indicate the coverage distance corresponding
to the 15 and 24.5 dBi horn antennas. The maximum coverage distance increases with increasing
antenna gains and a decrease of the PLE. For example, the radio waves can propagate about 200
m in a highly obstructed environment with a PLE of 5.76 when the combined TX-RX antenna
gain is 49 dBi, which closely agrees with Rappaports measured values. This indicates that the
coverage region of a base station can be enlarged by increasing antenna gains when in LOS
conditions (Rappaport et al., 2013, p. 342-3).
Figure 5: 28GHz maximum coverage distance with 119 dB maximum PL
Angle-of-Arrival (AOA) and Angle-of-Departure (AOD) Analysis
The purpose of AOA and AOD is to determine the multipath angular spread at the TX
(AOD) and RX (AOA). To determine the angles with the highest receive power a 360° sweep of
the TX and RX was conducted. Data collected at LOS, partially obstructed LOS, and NLOS
form the basis for the development of a spatial channel model. The path loss and root mean
squared (RMS) delay spreads are used to accurately characterize the channel. The figure below

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shows a polar plot of received power at the RX in a NLOS environment. The distance between
the TX and RX was 78 m. In the figure, each dot represents the received power level in dBm at
the corresponding RX azimuth angle. The number of resolvable multipath components, path loss
in dB with respect to the 5 m free space reference, and RMS delay spread in nanoseconds are
displayed from left to right on the outside of the plot. On the figure it can be seen that 22 TX-RX
links were successful out of 36 RX azimuth angles. Moreover, many multipath components exist
at numerous different pointing angles, providing great diversity which can be utilized for beam
combining and link improvement in future 5G systems.
Figure 6: Polar plot showing the received power at NLOS locations
Small scale fading was also explored by moving the RX at half-wavelength (5.35 mm)
increments along a small scale linear track of 10 wavelengths (107 mm). The figure below shows
the 3D PDPs of small scale fading for the TX-RX angle combination for the strongest received
power. The maximum and minimum received signal powers were -68 dBm/ns and -74 dBm/ns,
respectively, yielding merely ±3 dB fading variation. This indicates that movements over the
small scale track exert little influence on the AOA or the received power level of multipath
signals (Rappaport et al., 2013, p. 343-4).

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Figure 7: Power delay profile measured over 10-wavelength linear rack at 28 GHz
38 GHz
The experiments at 38 GHz took place in the smaller urban environment of Austin,
Texas. In this location, an 800 MHz null-to-null bandwidth spread spectrum sliding correlator
channel sounder was used. The PN sequence was operating at 400 Mcps and 399.9 Mcps at the
TX and RX, respectively, to provide a slide factor of 8000 and adequate processing gain. The PN
sequence was modulated by a 5.4 GHz IF signal and was up-converted by a LO to 37.625 GHz
with a +22 dBm output power before the TX antenna. A 25-dBi gain Ka-band vertically
polarized horn antenna with 7.8° half-power beam width was used at the TX, and an identical
antenna with a wider beam 13.3 dBi gain and 49.4° beam width vertically polarized horn
antenna was used at the RX. The maximum measurable path loss was about 160 dB.
A total of 43 TX-RX combinations were measured with up to 12 antenna configurations
for each location. As with the 28 GHz experiments, the RX were positioned in a number of LOS,
partially obstructed LOS, and NLOS locations. At each RX location, measurements were
acquired using a circular track with 8 equally spaced local area points separated by 45°. The
radius of the track had a 10 wavelength separation distance between consecutive points along
the track. For LOS links, the TX and RX were pointed directly at each other in both azimuth and
elevation. The captured PDPs for each complete track measurement were then averaged and a
new RX location was selected. NLOS conditions were taken over the track and a subsequent
360° azimuth signal search was performed
13.3 dBi and 25 dBi horn antennas were used to determine path loss. The LOS PLE for
the 25 dBi horn antennas was 2.30 and NLOS PLE was 3.86. Like 28 GHz, atmospheric

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absorption is negligible at 200 m. It is 0.016 dB over 200 m. There were no results provided for
rain. (Rappaport et al., 2013, p. 338)
AOA measurements were most common when the RX azimuth angle was between -20°
and +20° about the bore sight of the TX azimuth angle. Based upon the data of the locations of
the RX and their corresponding TX, a lower base station height is more likely to have more links
with varying TX azimuth angle. However, the location of the RX impacts the observed AOA and
multipath response.
The RMS delay spread showed sensitivity to antenna gains. The cumulative distribution
functions (CDF) for LOS and NLOS links are similar. This shows that a lower antenna gain has a
higher RMS delay spread, whereas the 25 dBi antenna showed lower delays with greater TX-RX
separation. The graph below shows RMS delay spreads for the 25 dBi and 13.3 dBi RX antennas
plotted as a function of arc length.
Figure 8: RMS delay spread as a function of arc length at 38 GHz
In an outage study, it was determined that lower base station heights provide better close-
in coverage. By comparing 36 and 18 meters high base stations, the authors found that no
outages occurred within a 200 m cell radius. At more than 200 m distance, 52.8% of locations
were outages, 10% of those belong to one location, and 27.3% to another location. The coverage
radius of 200 m is similar to NYC which means that 200 m is a viable cell size for future 5G
systems. (Rappaport et al., 2013, p. 344-5).

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60 GHz
Much of the research about 60 GHz involves indoor WLAN systems. Measured indoor
LOS PLEs are 1.3 in corridors, 1.7 in a laboratory, and 2.2 in an office area. NLOS PLEs ranged
from 3.0 to 3.8 in typical office environments. Average RMS delay spreads were 12.3 ns and
14.6 ns in LOS and NLOS environments respectively (Deng, Samimi, & Rappaport, 2015, p. 1).
In another paper, Niu et al. (2015) say the free space propagation loss at 60 GHz is proportional
to the square of the carrier frequency. This means that for a wavelength of about 5 mm, the free
space propagation loss is 28 dB more than it is at 2.4 GHz. Atmospheric absorption ranges
between 15 and 30 dB/km. Large scale fading F(d) can be modeled by the following equation.
where PL(d0) is the path loss at distance d0, n is the path loss exponent, Sσ is the showing loss,
and σ is the standard deviation of Sσ. The table below lists values of these parameters for a
corridor and LOS/NLOS for a hall.
Table 3: 60 GHz parameters for path loss model
For the small-scale propagation effects, the multipath effect is not obvious with
directional antennas. By using circular polarization and receiving antennas of narrow beam
width, multipath reflection can be suppressed (p. 2-3).
In the LOS channel model in the conference room environment, the direct path contains
almost all the energy, and nearly no other multipath components exist. In this case, the channel
can be regarded as Additive White Gaussian Noise (AWGN). In the NLOS channel, there is no
direct path, and the number of paths with significant energy is small. To achieve high data rate
and maximize the power efficiency, mmWave communications mainly rely on the LOS
transmission (Niu et al., 2015, p. 3).

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73 GHz
Like the 60 GHz experiments, the 73 GHz tests were conducted indoors. The setup
involved using a 400 Megachip-per-second (Mcps) spread spectrum broadband sliding correlator
channel sounder, and two pairs of 15 dBi 30° half-power beam width (HPBW) and 20 dBi 15°◦
HPBW high gain directional antennas for the TX and RX. The TX and RX were placed 2.5 m
and 1.5 m above ground level respectively to replicate a typical WLAN environment. Five TX
and 33 RX locations were tested with separation distances between 3.9 m to 45.9 m in an office
with various obstructions. For each TX-RX combination, 8 different pointing angle measurement
sweeps were performed at both the TX and RX to determine AOD and AOA statistics, and a
power delay profile (PDP) was acquired at each azimuth and elevation angle in steps of 15° or
30° depending on the carrier frequency. All azimuth sweeps were performed in both vertical-to-
vertical (V-V) and vertical-to-horizontal (V-H) antenna polarization scenarios (Deng et al., 2015,
p. 1-3). The following table from the Deng study shows the parameters for the path loss
parameters for 28 GHz and 73 GHz using V-V polarization. As expected, the PLE and σ are
worse for the 73 GHz system except for directional polarization with a LOS where the PL is the
same at 1.7 and σ is slightly better at 2.1 versus 2.6.
Table 4: 28 and 73 GHz PLEs and standard deviations for directional and omnidirectional PL models and for co-polarization
V-V cross polarization
The following table shows the parameters for the path loss parameters for 28 GHz and 73
GHz using V-H polarization. As expected, the 73 GHz parameters are worse than the 28 GHz
parameters.

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Table 5: 28 and 73 GHz PLEs and standard deviations for both directional and omnidirectional path loss models and for cross-
polarization V-H scenarios
The following table shows mean, standard deviation, and maximum values of mean RMS
delay spread for 28 GHz and 73 GHz using V-V and V-H polarization.73 GHz does better on V-
V LOS and V-H NLOS and does worse in LOS V-H and NLOS V-V.
Table 6: 28 and 73 GHz mean RMS delay spread (ns) for co- and cross-polarization combinations in LOS and NLOS scenarios
Millimeter Wave Challenges
In their paper, Niu et al. (2015) identified four mmWave challenges within the office
environment. They are integrated circuits and system design, interference management and
special reuse, anti-blockage, and dynamics due to user mobility. There are discussed in the
subsections below with additional information from other research papers.
Integrated Circuits and System Design
In the 60 GHz band, high transmit power and huge bandwidth cause severe nonlinear
distortion of power amplifiers (PA). Phase noise and IQ imbalance are also challenging problems
faced by radio frequency (RF) integrated circuits (Niu et al., 2015, p. 5). Also, the high power
consumption of analog-to-digital (ADC) and digital-to-analog (DAC) converters is an issue.

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Therefore, the current architecture where every antenna is connected to a high-rate ADC/DAC is
not likely to be applicable to mmWave unless there is a significant improvement in
semiconductor technology (Boccardi et al., 2014, p. 4).
Interference Management and Spatial Reuse
In the outdoor 60 GHz mesh network, the directional links are modeled as pseudo-wired,
and the interference between nonadjacent links is negligible. As a result of the directional
transmission, the third party nodes cannot perform carrier sense as in WiFi, which is referred to
as the deafness problem (this will appear several times later in this report). Therefore, the
coordination mechanism becomes the key to the MAC design, and concurrent transmission
should be used to enhance network capacity.
In the indoor environment, due to the limited range, the assumption of pseudo-wired
doesn’t hold. Due to the exponential growth in mobile data and the short range of mmWave
communication, the number of deployed access points (APs) increases tremendously. For
example, a large number of APs must be deployed in scenarios such as enterprise cubicles and
conference rooms to provide seamless coverage. In this case, the interference in the network can
be divided into two portions: interference within each business service set (BSS), and
interference among different BSSs. This is shown in the figure below. When the two links in
BSS1 and BSS2 are communicating in the same slot t and AP1 directs its beam towards the
laptop, AP2 will have interference to the laptop. If the distance between them is short, the service
of the laptop will be dramatically degraded.
As a result of the above scenario, interference management schemes such as power
control and transmission coordination should be applied to avoid major degradation of network
performance. With interference efficiently managed, concurrent transmission (spatial reuse)
could be supported among different BSSs as well as within each BSS (Niu et al, 2015, p. 6).

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Blockage
As already discussed in the previous section, mmWave suffers from huge propagation
loss and beam forming (BF) is an essential technique to directionally steer antennas so base
stations are within LOS of each other. Also, mmWave has weak diffraction ability which makes
communication sensitive to blockage by obstacles such as humans and furniture (Niu et al.,
2015, p. 2). For example, blockage by a human penalizes the link budget by 20–30 dB. Results
from a propagation study in an indoor environment involving people show that the channel is
blocked for about 1% or 2% of the time for one to five persons. (Niu et al., 2015, p. 4).
Also, a pair of researchers, Sato and Manabe, estimated the propagation path visibility
between APs and terminals in office environments where there is blockage by people. To avoid
shadowing by people without using multi-AP diversity, a lot of APs are needed. However,
diversity switching between only two APs provides 98% propagation path visibility. Dong et al.,
another group of researchers, analyzed the link blockage probability in typical indoor
environments under random human activities. The results they obtained show that as the user
devices move towards the edge of the service area, the blockage probability of links increases
almost linearly (Niu et al, 2015, p. 7-8).
User Mobility
User mobility presents two challenges in the mmWave system. First, mobility will incur
significant changes of the channel state. When users move, the distance between the TX and the
RX varies, and the channel state also changes accordingly. The table below lists the channel
capacities under difference distances between TX and RX assuming LOS transmission. The
capacities were calculated according to Shannon’s channel capacity. From the table, the channel
capacity decreases from 16.02 Gbps at 1 m to 4.75 Gbps at 10 m from the TX. Therefore, it is
imperative that the selection of modulation and coding schemes (MCS) should be selected
according to the channel states to fully exploit the potential of mmWave communications.
Table 7: Channel capacities under different distances
The second challenge is due to the small coverage areas of BSSs. User mobility causes
significant and rapid load fluctuations in each BSS. Also, maintaining a reliable connection for

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delay sensitive applications such as HDTV is a big challenge. Therefore, user association and
handovers between APs should be carried out intelligently to achieve an optimized load balance.
(Niu et al., 2015, pp. 4 & 8)
Millimeter Wave Solutions
Wireless Backhaul
Pi et al. came up with a solution for wireless backhaul called mmWave Gbps Broadband
(MGB). As can be seen in the figure below, their solution may lead to a convergence of LTE,
Wi-Fi, and 5G.
Figure 9: MGB Backhaul
In this design, both the TX and RX use a hybrid of analog beam forming and digital
MIMO processing to adapt to channel condition and balance analog power consumption and
digital processing complexity. A typical MGB hub has 3 sectors. Each sector uses a planar
phased antenna array and dynamically form beams to transmit to and receive from small cells.
The small cells also use planar phased antenna arrays to point to the best directions to transmit to
and receive from the MGB hub. On the access link, small cells can use either LTE, or Wi-Fi, or
5G, or the combination of these access technologies to communicate with a variety of devices
such as smart phones, tablets, laptops, etc.
With multiple small cells within its coverage area, the MGB hub can use multi-user
MIMO to communicate with multiple small cells simultaneously when needed. In addition, Time
Division Multiple access (TDMA) and Orthogonal Frequency Division Multiple Access
(OFDMA) are also supported in multiplexing traffic to and from small cells. To boost mobile or
fixed broadband throughput, Time Division Duplex (TDD) is the preferred duplex scheme

18
because of its flexibility to adapt to the asymmetry of uplink and downlink data traffic, and to
maximize throughput.
The link budget for a proposed MGB system at 39 GHz with 500 MHz system bandwidth
and cell radius of 1 km is shown in the table below. (Pi et al., 2015, p. 2)
Table 8:MGB wireless backhaul link budget
The MGB hub uses a 256-element antenna array and 64 PAs with 10 dBm output power
each. The small cell uses a 64-element antenna array and 16 PAs with 10 dBm output power
each. Effective Isotropic Radiation Power (EIRP) of 55.14 dBm and 43.10 dBm can be achieved
for downlink and uplink, respectively. The path loss is modeled by free space loss plus an
additional loss of 15 dB per km to account for other factors such as rain, reflection, atmospheric
absorption, etc. More than 1 Gbps can be achieved in both the downlink and uplink at the cell
edge 1 km from the hub. With 4-stream multi-user MIMO to 4 small cells with median path loss
707 meters from the hub, 7.7 Gbps throughput can be achieved per sector in both the downlink
and uplink
The radius of an MGB system needs to be large enough to provide sufficient coverage,
yet small enough to provide Gbps connectivity with great availability. The author recommends

19
the MGB cell radius to be in the range of 300 m – 3 km. A cell radius greater than 3 km leads to
significant degradation of performance at the cell edge based upon the link budget above. A cell
radius that is smaller than 300 m has little footprint for Gbps backhaul which makes it difficult to
justify the cost of the system. For example, for a MGB cell radius of 1 km, less than 500 hubs are
needed to cover the whole New York City (Pi et al., 2015, p. 3).
Integrated Circuits and System Design
With respect to the high power used by ADCs and DACs, one alternative is a hybrid
architecture where beam forming is performed in analog at RF and multiple sets of beam formers
are connected to a small number of ADCs or DACs. In this scenario, signal processing
algorithms are needed to steer the analog beam forming weights. Another alternative is to
connect each RF chain to a 1-bit ADC/DAC with very low power requirements. In this scenario,
the beam forming would be performed digitally but on very noisy data (Boccardi et al., 2014, p.
4).
Although Niu et al. does not mention specifics, they do state that research on integrated
circuits for the 60 GHz band includes on-chip and in-package antennas, radiofrequency (RF)
power amplifiers (PAs), low-noise amplifiers (LNAs), voltage-controlled oscillators (VCOs),
mixers, and analog-to-digital converters (ADCs).
However, with respect to antenna designs, they mention several alternatives. Hong et al.
created a phased array antenna solution operating at 28 GHz with near spherical coverage. They
also designed cellular phone prototype equipped with mmWave 5G antenna arrays consisting of
a total of 32 low-profile antenna elements. Hu et al. developed a cavity-backed slot (CBS)
antenna for millimeter-wave applications. The cavity of the antenna is fully filled by polymer
material, which reduces the cavity size by 76.8%. Liao et al. created a planar aperture antenna
with differential feeding, which maintains a high gain and wide bandwidth compared with
conventional high gain aperture antennas. Their proposed aperture antenna element has low cost,
low profile, compact size, and is also good in gain and bandwidth. Zwick et al. created a new
planar superstrate antenna suitable for integration with mmWave transceiver integrated circuits,
which is printed on the bottom of a dielectric superstrate with a ground plane below. Two of their
designs for the 60 GHz band achieve over 10% bandwidth while maintaining better than 80%
efficiency. (Niu et al., 2015, p. 5-6)

20
Interference Management and Spatial Reuse
In order to address challenges with interference management and special reuse, there has
been some research on directional MAC protocols for mmWave communications. Many of the
proposed protocols are based on TDMA since it is specified in several standards such as ECMA-
387, IEEE 802.15.3c, and IEEE 802.11ad. Cai et al. introduced the concept of exclusive region
(ER) to enable concurrent transmissions, and derived the ER conditions that concurrent
transmissions always outperform TDMA for both omni-directional antenna and directional-
antenna models. By using the REX scheduling scheme, significant spatial reuse gain is achieved.
However, the interference level and the received signal power are calculated by the free space
path loss model, which is not valid for indoor wireless personal area networks (WPANs), where
reflection will also cause interference. Other problems with this scheme are that it only considers
two-dimensional space in the transmission scheduling problem, and the power control is not
considered to manage interference.
In two protocols based on IEEE 802.15.3c, multiple links are scheduled to communicate
in the same slot if the multi-user interference (MUI) is below a specific threshold. However, they
do not capture the characteristics of the directional antennas or the effect of interference from
multiple links.
Qiao et al. developed a concurrent transmission scheduling algorithm for WPAN where
non-interfering and interfering links are scheduled to transmit concurrently to maximize the
number of flows with the QoS requirement of each flow satisfied. The advantage of this
approach is that it can support more users and significantly improves the resource utilization
efficiency in WPANs. On the other hand, it does not consider NLOS transmissions, and the
interference model does not take the antenna model into account.
Based on IEEE 802.11ad, Chen et al. proposed a spatial reuse strategy to schedule two
different service points (SPs) to overlap with each other, and also analyzed the performance of
the strategy with the difference between idealistic and realistic directional antennas considered.
A drawback is that it does not fully exploit the spatial reuse since only two links are considered
for concurrent transmissions.
Gong et al. proposed a directive CSMA/CA protocol, which exploits the virtual carrier
sensing to solve the deafness problem (see page 15). The network allocation vector (NAV)

21
information is distributed by the piconet controller (PNC). Spatial reuse, however, is not fully
exploited to improve network capacity in the protocol.
Son et al. proposed a frame-based directive MAC protocol (FDMAC). The high
efficiency of FDMAC is achieved by amortizing the scheduling overhead over multiple
concurrent transmissions in a row. The core of FDMAC is the Greedy Coloring algorithm which
fully exploits spatial reuse and greatly improves the network throughput compared with
MRDMAC and memory-guided directional MAC (MDMAC). FDMAC also has a good fairness
performance and low complexity. FDMAC, however, assumes the pseudo-wired interference
model for WPANs, which is not reasonable due to the limited range.
Chen et al. proposed a directional-cooperative MAC protocol (DCoopMAC) to
coordinate the uplink channel access among stations in a WLAN. In D-CoopMAC, a two-hop
path of high channel quality from the source station (STA) to the destination station (STA) is
established to replace the direct path of poor channel quality. By the two-hop relaying, D-
CoopMAC significantly improves the system throughput. However, spatial reuse is also not
considered in D-CoopMAC since most transmissions go through the access point (AP).
Park et al. proposed an incremental multicast grouping (IMG) scheme to maximize the
sum rate of devices, where adaptive beam widths are generated depending on the locations of
multicast devices. Simulations demonstrate that the IMG scheme can improve the overall
throughput by 28–79% compared with the conventional multicast schemes.
Scott-Hayward and Garcia-Palacios proposed to use particle swarm optimization (PSO)
for the channel-time allocation of a mixed set of multimedia applications. Channel-time
allocation PSO (CTA-PSO) is demonstrated to allocate resource successfully even when
blockage occurs.
For outdoor mesh networks in the 60 GHz band, Singh et al. proposed a distributed MAC
protocol, the memory-guided directional MAC (MDMAC), based on the pseudo-wired link
abstractions. A Markov state transition diagram is incorporated into the protocol to alleviate the
deafness problem (see page 15). MDMAC employs memory to achieve approximate time
division multiplexed (TDM) schedules, and does not fully exploit the potential of spatial reuse.
Another distributed MAC protocol for directional mmWave networks is directional-to-
directional MAC (DtDMAC), where both senders and receivers operate in a directional-only
mode. DtDMAC adopts an exponential back-off procedure for asynchronous operation, and the

22
deafness problem (see page 15) is also alleviated by a Markov state transition diagram.
DtDMAC is fully distributed, and does not require synchronization. However, it does not capture
the characteristics of wireless channel in mmWave bands, and only gives the analytical network
throughput of DtDMAC for the mmWave technology (Niu et al., 2015, p. 6-7).
Blockage
To ensure robust network connectivity, different approaches from the physical layer to
the network layer have been proposed. Genc et al. use reflections from walls and other surfaces
to steer around obstructions. Likewise, Yiu and Singh used static reflectors to maintain the
coverage in the entire room when blockage occurs. However, using reflections will cause
additional power loss and reduce power efficiency. The node placement and environment will
have a big impact on the efficacy of reflection to overcome blockage. An et al. resolved link
blockage by switching the beam path from a LOS link to a NLOS link. However, NLOS
transmissions suffer from significant attenuation and cannot support high data rate.
Park and Pan proposed a spatial diversity technique, called equal-gain (EG) diversity
scheme, where multiple beams along the N strongest propagation paths are formed
simultaneously during a beam forming process. When the strongest path is blocked by obstacles,
the remaining paths can be used to maintain reliable network connectivity. This approach adds
complexity and overhead of the beam forming process and will degrade the system performance
over time.
Xiao proposed a suboptimal spatial diversity scheme called maximal selection (MS) by
tracing the shadowing process. This scheme outperforms EG in terms of link margin and saves
computation complexity. Another approach is to use relays to maintain the connectivity. The
multi-hop-relay directional MAC (MRDMAC) overcomes the deafness problem (see page 15) by
PNC’s weighted round robin scheduling. In MRDMAC, if a wireless terminal (WT) is lost due to
blockage, the access point (AP) will choose a WT among the live WTs as a relay to the lost
node. By the multi-hop MAC architecture, MRDMAC is able to provide robust connectivity in
typical office settings. Since most transmissions go through the PNC, concurrent transmission is
also not considered in MRDMAC.
Based on IEEE 802.15.3c, Lan et al. use two-hop relaying to provide alternative
communication links under such harsh conditions. The transmission from relay to destination of
one link is scheduled to coexist with the transmission from source to relay of another link to

23
improve throughput and delay performance. However, only two links are scheduled for
concurrent transmissions in this scheme, and the spatial reuse is not fully exploited.
Lan et al. proposed a deflection routing scheme to improve the effective throughput by
sharing time slots for direct path with relay path. It includes a routing algorithm, the best fit
deflection routing (BFDR), to find the relay path with the least interference that maximizes the
system throughput. They also developed the sub-optimal random fit deflection routing (RFDR),
which achieves almost the same order of throughput improvement with much lower complexity.
With multiple APs deployed, handovers can be performed between APs to address the blockage
problem.
Zhang et al. use multi-AP diversity to overcome blockage. When one wireless link is
blocked another AP can be selected to complete remaining transmissions. To ensure that this
approach works multiple APs need to be deployed. Their locations will have a big impact on the
robustness and efficiency of this approach.
Niu et al. proposed a blockage robust and efficient directional MAC protocol
(BRDMAC), which overcomes the blockage problem by two-hop relaying. In BRDMAC, relay
selection and spatial reuse are optimized jointly to achieve near-optimal network performance in
terms of delay and throughput. However, only two-hop relaying is considered in BRDMAC, and
under serious blockage conditions, there is probably no two-hop relay path between the sender
and the receiver. In the network layer, Wang et al. exploited multipath routing to enhance
reliability of high quality video in the 60 GHz radio indoor networks. It mainly focuses on the
video traffic so other traffic patterns are not considered (Niu et al., 2015, p. 8).
User Mobility
Current standards for mmWave communications, such as IEEE 802.11ad and IEEE
802.15.3c, adopt the received signal strength indicator (RSSI) for user association, which may
lead to inefficient use of resources. With load, channel quality, and the characteristics of 60 GHz
wireless channels taken into consideration, Athanasiou et al. designed a distributed association
algorithm (DAA), based on Langragian duality theory and subgradient methods. DAA is shown
to be asymptotically optimal, and outperforms the user association policy based on RSSI in terms
of fast convergence, scalability, time efficiency, and fair execution. Also, user mobility will
encounter frequent handovers between APs. Handover mechanisms have a big impact on QoS
guarantee, load balance, and network capacity, etc. Smooth handovers are needed to reduce

24
dropped connections and ping-pong (multiple handovers between the same pair of APs).
However, there is little work on the handover mechanisms for mmWave communications in the
60 GHz band. Van Quang et al. discussed the handover issues in radio over fibre network at 60
GHz, and the handover performance can be improved using more information such as velocity
and mobility direction of users. Tsagkaris et al. proposed a handover scheme based on Moving
Extended Cells (MEC) to achieve seamless broadband wireless communication (Niu et al., 2015,
p. 9).
Conclusion
The current 4G will be under tremendous pressure in the next few years from the
onslaught of increased mobile data traffic and other technologies such as vehicle-to-vehicle
communication. The bandwidth within the microwave frequencies will not be enough to meet
future network demands. This has led to a lot of research into the next generation technology–
5G. Many of the details of 5G have yet to be worked out, but the goals are a thousand-fold
increase in capacity, hundred-fold increase in energy efficiency, and tens of lower latency. Many
of the details have yet to be worked out, but some of the technologies include: mmWave,
massive MIMO, device-centric architecture, machine-to-machine communication, and smarter
devices. Many experts expect 5G to be deployed around 2020.
The mmWave spectrum spans from 30 to 300 GHz. The bands of interest are LMDS
which consists of sub-bands ranging from 27.5 to 31.3 GHz, 39 GHz band (38.6 to 40.0 GHz),
V-band (57-67 GHz), and E-band which consists of several sub-bands in the 70, 80, and 90 GHz
ranges. Two mmWave applications were discussed as well. They include small cell access and
wireless backhaul.
The next major section discussed mmWave channel characteristics of the 28, 38. 60, and
73 GHz bands. Although mmWave suffers from high path loss, the 28 and 38 GHz bands are
ideal for small cells of 200 m or less. 60 and 73 GHz frequencies are primarily used for indoor
environments. One consequence of path loss is there needs to be a repeater or access point
between the inside and outside environment to handoff signals as the waves cannot penetrate
tinted glass and brick. Also, for 28 and 38 GHz frequencies, atmospheric absorption and heavy
rain are negligible up to 200 m which, once again, makes them ideal for small cells. Also outside
the 200 m radius, the outage probability increases dramatically. High path loss is not all bad

25
because it reduces inter-cell interference and allows frequency reuse. Path loss exponents were
also presented for the various frequencies.
Finally several challenges and solutions involving mmWave were discussed. Challenges
included integrated circuits and system design, interference management and special reuse,
blockage, and user mobility. For integrated circuits and system design the main challenge is that
the high transmit power and huge bandwidth cause nonlinear distortion in the PA. Also, phase
noise and IQ imbalance are issues as well as high power consumption in the ADC and DAC
converters. Solutions for ADCs and DACs issue are to use an alternative architecture for beam
forming or connect each RF chain to 1 bit ADC/DAC. Other areas of research were presented
such as on-chip or in-package antennas, RF, PA, LNAs, VOCs, etc. Several antenna designs
were discussed such as phased antenna, cavity antenna, aperture antenna, and planar superstrate
antenna.
Some of challenges regarding interference include coordination mechanisms and
interference between BSSs. Many solutions were explored such as Exclusive Region, REX
scheduling scheme, concurrent transmission scheduling algorithms, etc.
Another challenge is blockage from people in an office environment. To combat this
some solutions involve using reflection from walls or other objects but these are subject to high
path loss. Another solutions use a spatial diversity scheme. They include equal-gain which finds
the strongest links between two positions and automatically switches links when the path gets
congested. Another is called maximal selection which traces the shadow process.
The last challenge is user mobility. As users move the channel state changes and users
will get less bandwidth as they move further away from the TX. Another challenge is handover
between APs to optimize load balance. Some solutions discussed include using distributed
association algorithm for user association and using a handover scheme based on Moving
Extended Cells to achieve seamless wireless broadband.
Finally, a wireless backhaul solution called MGB was discussed. It uses a hybrid of
analog beam forming and digital MIMO processing coupled with planar phased arrays to provide
multi-gigabit per second data rates.

26
References
Boccardi, F., Heath, R., Marzetta, T., Popovski, P., & Lozano Solsona, A. (2014). Five
disruptive technology directions for 5G.
Deng, S., Samimi, M., & Rappaport, T. (2015). 28 GHz and 73 GHz Millimeter-Wave Indoor
Propagation Measurements and Path Loss Models.
Gao, Z., Dai, L., Mi, D., Wang, Z., Imran, M., & Shakir, M. (2015 MmWave Massive MIMO
Based Wireless Backhaul for 5G Ultra-Dense Network.
Niu, Y., Li, D., Jin, L., Su, A., & Vasilakos, V. (2015). A survey of millimeter wave
communications (mmWave) for 5G: Opportunities and challenges. WirelessNetworks,
21(8), 2657-2676.
Pi, Z., Choi, J., & Heath Jr, R. (2015). Millimeter-wave Gbps Broadband Evolution towards 5G:
Fixed Access and Backhaul.
Rappaport, Shu Sun, Mayzus, Hang Zhao, Azar, Wang, . . . Gutierrez. (2013). Millimeter Wave
Mobile Communications for 5G Cellular: It Will Work! Access, IEEE,1, 335-349.
Swindlehurst, A., Ayanoglu, E., Heydari, P., & Capolino, F. (2014). Millimeter-wave massive
MIMO: The next wireless revolution? Communications Magazine, IEEE, 52(9), 56-62.

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