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

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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)

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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

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