A Primer on 5G

Ing. Francesco Amato, Ph.D

January 11, 2019

Abstract

The fifth generation (5G) wireless communication systems pledge toprovide a massive amount of raw bandwidth, low latency, and multigigabit-per-second (Gbps) data rates through both the sub-6 GHz bands (e.g.: 5.8GHz) and the mmWave frequency spectrum. This document aims in giv-ing the reader a broad overview about the physical layer underlying the5G technology; it summarizes relevant research publications and providesa useful bibliography for those who want to approach, understand anddeepen their knowledge on 5G.

1 Engineering Requirements for 5G

ITU-R1 have defined three main types of usage scenario that the capabilityof 5G New Radio (NR) is expected to enable. They are Enhanced MobileBroadband (eMBB), Ultra Reliable Low Latency Communications (URLLC),and Massive Machine Type Communications (mMTC). eMBB refers to using5G as an evolution to 4G LTE mobile broadband services with faster connectionwith higher throughput and more capacity. 5G will use spectrum in the existingLTE frequency range (600 MHz to 6 GHz)2 and also in mmWave bands (24–86GHz)3.

Initial 5G launches in the sub-6 GHz band will not diverge architecturallyfrom existing LTE 4G infrastructure. In the United States, the four majorcarriers have all announced deployments: AT&T’s millimeter wave commercialdeployments in 2018 (28/39 GHz for fixed wireless), Verizon’s 5G fixed wirelessat 28 GHz launches in four U.S. cities and millimeter-wave deployments, Sprint’slaunch in the mobile 2.5 GHz band, and T-Mobile’s mobile 600 MHz 5G launchin 30 cities.

MmWave frequencies have been explored in the past; some of the first ex-periments by Bose and Lebedev were performed in 1890s in the mmWave bandand the fist standardized consumer radios were in the 60 GHz unlicensed band.

1International Telecommunication Union-Radiocommunication Sector2In Sub 6-GHz bands, 5G is evolutionary since it will not diverge architecturally from

existing LTE 4G infrastructure. At this band, LTE max modulation format is 128 QAM,while in sub-6 GHz a modulation format at 256 QAM is supported resulting in a significantthroughput improvement at sub-6 GHz bands. Nevertheless, LTE-Advanced already uses 256QAM.

3Promising bands are 28–30 GHz; the license-free band at 60 GHz; and the E-band at71–76 GHz, 81–86 GHz, and 92–95 GHz. At the 60 GHz band, different spectra are allocateddepending on countries (e.g. 57 – 66 GHz in Europe; 57 – 64 GHz in North America andSouth Korea; 59.4 – 62.9 GHz in Australia; 59–66 GHz in Japan.)

1

Moreover, channels of 2 GHz of BW are common for systems operating in the60 GHz unlicensed band. It seems that WLAN and high speed wearable (PAN)devices connected to cell phones, smart watches, augmented reality glasses andvirtual reality headsets operating at 60 GHz will be the first widely deployedconsumer wireless devices at mmWave. Since mmWave are already used for au-tomotive radars, they could play a key role in developing connected autonomouscars.

Requirements for 5G technology are the following4:

• Data rates: peak data rates of 20 Gbps and 1 Gbps user experienced.That is, 1000X or 100X the current 4G technology;

• Latency. Current 4G technology latencies are on the order of 15 ms. Two-way gaming, tactile internet, virtual and enhanced realities will need 5Gto support a roundtrip latency of 1 ms;

• Energy and costs: the same energy efficiency of 4G is expected. Ideally,energy consumption should be reduced. Since it is expected a per-linkdata rate increase by about 100X, a Joules per bit and cost per bit willneed to fall by 100X, at least.

Among those requirements, the one that gets more attention is the need forhigher data rates. High data rates can be achieved by combining three solutions:

• more active nodes per unit area. That is, more cells;

• increased bandwidth. Achievable by both moving into the mmWave spec-trum and making better use of WiFi’s unlicensed spectrum in the sub-6GHz band;

• a spectral efficiency 3X to 4X that of 4G is expected to support morebits/s/Hz. Achievable my using advanced MIMO architectures.

1.1 More Cells

The first generation of cells, in the early 1980s, had cell sizes of hundreds ofsquare km. Nowadays, in Japan, the spacing between base stations can be assmall as two hundred meters, corresponding to a cell size under the tenth ofsquare km.

1.2 Increasing the Bandwidth Using mmWave Bands

So far, the mmWave spectrum was left idle because it was considered unsuitablefor mobile communications due to hostile propagation qualities, strong pathloss, atmospheric and rain absorption, low diffraction around obstacles, higherscattering due to increased effective roughness of materials, larger penetrationlosses through objects, and, additionally, because of strong phase noise andhigh equipment costs. Nonetheless, semiconductors are maturing, their costsand power consumption are rapidly falling, and experimental setups are beingdeveloped in academic settings [2].

4Most of the info in this section are a summary of what can be found in [1].

2

Although atmospheric and rain absorption is high, especially the 15dB/km oxygen absorption at the 60 GHz band, this is inconsequential for bothshort-range indoor links and for the urban cellular envisioned deployments wherethe BS spacing is of the order of 200 m. In this case, the absorption is actuallybeneficial to reduce interference from more distant BS.

Since mmWave shows reduced diffraction and more specular propagationthan microwaves, they are more prone to blockages: as the transmit-receivedistance grows, the pathloss drops to 40 dB/decade plus additional blockingloss of 15-40 dB when no LoS is assured. A brick can attenuate mmWave sig-nals by 40 to 80 dB; the human body itself can result in 20 to 35 dB loss;foliage loss can also be significant. Nevertheless, both the human body andmost building materials are reflective; allowing them to be important scatterersthat enable coverage via NLOS paths. Blocking models can be derived an-alytically from random shape theory or from geographic information. Usingthese models, it is possible to analytically evaluate coverage and capacity inmmWave cellular networks using stochastic geometry. Because of blocking, alink can easily transition from usable to unusable and, differently from small-scale fading, large-scale obstructions cannot be solved with small-scale fadingcountermeasures. In mmWave, therefore, interference is de-emphasized and thewide bandwidth of operation might make them often noise-limited (rather thaninterference-limited like in 4G). New channel models capturing these effects arecurrently being developed.

It is often assumed that, at higher frequencies, signals incur a much higherpropagation loss in free space compared to lower frequencies; nevertheless,given the same physical aperture size, transmit and receive antennas at higherfrequencies send and receive more energy through narrower directed beams.Increasing the transmitting frequency fc by an order of magnitude, adds 20 dBof power loss regardless of the transmit-receive distance. However, if the antennaaperture is kept constant at one end of the link as the frequency increases, thenthe free-space path loss remains unchanged. If both the transmit and receiveantenna apertures are held constant, then the free-space path loss diminisheswith f2c .

It is possible to maintain the same effective aperture by using antennaarrays both at the transmitter and the receiver. Array size examples in theliterature include 16 or 256 elements and it can be even higher in a BS cellularsystem; IEEE 802.11ad products with 32 elements are already available. Witharrays, the challenge becomes to cophase many antennas in a rapidly changingchannel due, for example, to mobility, blocking, and changing in device orien-tation. Since the costs and power consumption of ADC and DAC convertersoperating at wide bandwidths are high, it is hard to have fully digital beam-formers. More likely are structures based on old-fashioned analog phase shiftersor hybrid structures where groups of antennas share a single ADC and DAC.

1.3 MIMO

MIMO5 embodies the spatial dimension of the communication that arises oncea multiplicity of antennas are available at BS and mobile devices [1].

5Further info about MIMO can be found in the special issue [3]

3

MIMO systems allow spatial data multiplexing, in particular when thechannel is such that the transfer functions between different transmit and receiveantenna pairs are largely independent. In these schemes, multiple parallel datastreams are transmitted simultaneously and in the same frequency band. Withsufficient multipath in the channel, these different streams can be separatedat the receiver because of their distinct spatial signatures allowing an higherspectral efficiency [4].

Early version of MIMO systems included: single-user MIMO (SU-MIMO),where the spatial dimensions are limited by the number of antennas that canbe accommodated on a mobile device. When each BS communicate with sev-eral users concurrently, the multiuser MIMO (MU-MIMO) takes advantage ofthe antennas of all the users allowing for more spatial dimensions. In coordi-nated multipoint (CoMP), BSs can cooperate and act as a single effective MIMOtransceiver turning interference into useful signal. In 2007, Marzetta [5] pro-posed to equip BSs with a number of antennas much larger than the number ofactive users per time-frequency signaling resource, putting the number of anten-nas per BS into the hundreds. This proposal was initially named large-scaleantenna systems, but it is now known as massive MIMO.

MIMO technology has already been standardized and is widely used incurrent commercial WLAN (IEEE 802.11n/ac) and cellular (IEEE 802.16e/m,3GPP cellular LTE and LTE Advanced) systems at sub-6 GHz frequencies.These standards support a small number of antennas (up to a maximum ofeight, although two is commonly used). The arrays used at mmWave tendto have more elements than lower frequency systems (32 to 256 elements arecommon), but still occupy a small physical size due to the small wavelength.

At microwave frequencies, all the signal processing action happens in thebaseband, as illustrated in Fig. 1. Essentially, MIMO at conventional frequen-cies is an exercise in digital signal processing. At higher carrier frequencies andsignal bandwidths, there are several hardware constraints that make it difficultto have a separate RF chain (PAs, LNAs, VCOs) and data converter for eachantenna. First, the practical implementation of the power amplifier (PA) or thelow noise amplifier (LNA), the RF connections to each antenna element, and allbaseband connections are very difficult at mmWave bands; these devices haveto be packed behind each antenna, and all the antenna elements are placed veryclose to each other to avoid granting lobes; this space limitation prevents fromusing a complete RF chain per antenna. Second, power consumption is also alimiting factor: (i) PA, ADCs or data interface cards connecting digital com-ponents to DAC/ADCs are power hungry devices especially at mmWave; (ii) adigital conversion stage per antenna leads to a large demand on digital signalprocessing, since many parallel gigasamples per second data streams have to beprocessed, with an excessive power consumption as well.

The hardware constraints have led to several mmWave specific MIMO ar-chitectures where signal processing is accomplished in a mixture of analog anddigital domains or where different design trade-offs are made with respect tonumber of antennas or resolution of data converters.

1.3.1 Analog Beamforming

Analog beamforming is one of the simplest approaches for applying MIMO inmmWave systems. It can be applied at both the transmitter and receiver.

4

Figure 1: Conventional MIMO architecture at frequencies below 6 GHz [6].

It is defacto solution supported in IEEE 802.11ad. Analog beamforming isoften implemented using a network of digitally controlled phase shifters. Inthis configuration, several antenna elements are connected via phase shiftersto a single RF chain, as illustrated in Fig. 2. The phase shifter weights areadaptively adjusted using digital signal processing using a specific strategy tosteer the beam and meet a given objective, for example to maximize receivedsignal power. The performance achieved with analog beamforming based onphased arrays is limited by the use of quantized phase shifts and the lack ofamplitude adjustment. This makes it more challenging to finely tune the beamsand steer nulls. Of course, analog beamforming with a single beamformer onlysupports single-user and single-stream transmission [6].

Figure 2: mmWave MIMO system using analog only beamforming [6].

1.3.2 Hybrid MIMO

Hybrid architectures are one approach for providing enhanced benefits of MIMOcommunication at mmWave frequencies. This architecture, shown in Fig. 3,divides the MIMO optimization process between analog and digital domains. Asmall number of transceivers are assumed (2 to 8), so that Ns < Lt < Nt andNr > Lr > Ns. Assuming that Ns > 1, then the hybrid approach allows spatialmultiplexing and multiuser MIMO to be implemented; analog beamforming isa special case when Ns = Lt = Lr = 1.

Figure 3: MIMO architecture at mmWave based on hybrid analog-digital pre-coding and combining [6].

5

The RF precoding/combining stage can be implemented using different ana-log approaches like phase shifters, switches or lenses. Two hybrid structures arepossible. In the first one, all the antennas can connect to each RF chain (asillustrated in Fig. 4a). In the second one (see Fig. 4b), the array can be dividedinto subarrays, where each subarray connects to its own individual transceiver.Having multiple subarrays reduces hardware complexity at the expense of lessoverall array flexibility. An alternative mmWave hybrid architecture that makesuse of switching networks with small losses further reduces complexity and powerconsumption (Fig. 4c− f). Every switch can be connected to all the antennasif the array size is small or to a subset of antennas for larger arrays.

Analog beamforming for Ns > 1 in the hybrid architecture can also berealized using a lens antenna at the front-end, using the fundamental fact thatlenses compute a spatial Fourier transform thereby enabling direct channel accessin beamspace. This MIMO transceiver architecture is illustrated in Fig. 5 andsuggests a practical pathway for realizing high dimensional MIMO transceiversat mmWave frequencies with significantly low hardware complexity comparedto conventional approaches based on digital beamforming.

Figure 4: Example of analog massive MIMO architectures [6].

2 Exposure to mmWaves

Interactions of mmWaves with the human body will be investigated i this sec-tion. Each remark will be followed by a short explanation.

Remark. mmWaves are nonionizing radiations6:

6Most of the info in this section are a summary of what can be found in [7, 8].

6

Figure 5: The CAP-MIMO transceiver that uses a lens-based front-end foranalog beamforming [6].

besides being seen as traveling electromagnetic waves, mmWave can also bedescribed as having a particle-like nature. In this case, each photon has anenergy level given by:

E = hf =hc

λ(1)

mmWaves energy E ranges from 0.1 to 1.2 meV ; this energy is not enough toremove electrons from an atom or a molecule, phenomenon linked to cancer;typically 12 eV is required.

2.1 Dosimetric Quantities to Measure RF Exposure

• Specific Absorption Rate:

SAR = Pdiss

m = σ|E|2ρ

[Wkg

]is a quantitative measure of RF power absorbed in a living body

• Plane-wave equivalent Power Density

PD = |Ei|22η =

η|H2i |

2

[Wm2

]in the near-field, the free space impedance η is function of position andit is different from the far field. PD might not be as useful as SARor temperature for assessing safety in mmWave devices operating in thenear-field.

• Transient temperature is useful to evaluate the effect of medium- and high-power radiation where skin can be easily heated. At microwave, UHF andbelow, RF radiation goes deep into the tissue. However, at mmWavefrequencies, most of the energy is adsorbed within the first few millimeterof human skin (e.g.: there is a penetration depth of 0.41 mm at 42.25GHz). Thermal injury due to overexposure of mmWaves should producesuperficial burns.

Remark. No dosimetric quantities are given for near-field mmWave exposure:

7

Figure 6: The FCC and ICNIRP PD restrictions for electromagnetic wave ex-posure from 10 MHz to 100 GHz [7].

Table 1: PD exposure limits to RF radiations in different countries.Gen. pub. (W/m2) Gen. pub. (mW/cm2) Freq. Range

ICNIRP 10 1 2-300 (GHz)FCC 10 1 1.5-100 (GHz)Italy 0.1 0.01 0.0001-300 (GHz)

For mmWaves, ICNIRP7 and FCC8 use PD as a basic restriction in themmWave exposure guidelines: 10 W/m2 for the general public and 50 W/m2

for the occupational groups (Fig. 6). Current governmental RF far-field PDexposure limits for whole-body exposure for the general public can be listed inTab. 1 Guidelines in Italy make it difficult to locate typical base station anten-nas on apartment buildings and other low structures near people.

Remark. Currently, regulations do not provide SAR exposure values for mmWaves:

4 W/kg is considered the whole-body SAR level threshold for when RFenergy begins to induce undesired biological effects on humans. FCC restrictswhole-body average exposure to a SAR of 0.4 W/kg for occupational exposurebetween 100 kHz and 6 GHz. A safety factor of five is introduced for the generalpublic (0.08 W/kg). It is generally accepted that the maximal localized SARcould be as high as 20 times the whole-body averaged SAR for the generalpublic: 1.6 W/kg in 1g of tissue (cube-shaped) for head and trunk (1-g SAR),4 W/kg in 10 g of tissue in limbs.

ICNIRP provides slightly different guidelines for maximum localized SAR

7International Commission on Non-Ionizing Radiation Protection8Federal Commission Communication

8

Figure 7: FCC compliance evaluation criteria used for different exposure sce-narios [7].

limitations: 2 W/kg in any 10g of (contiguous) tissue in the head and trunk and4 W/kg in any 10g of tissue in the limbs over 6 min of exposure for frequenciesup to 10 GHz for the general public. European and American regulations,therefore, differ for both the SAR values and the definition of tissue mass usedto define the SAR. At mmWave, where radiation should be more focused, ameasure of SAR over 10g of tissue would lower the SAR value by a factor oftwo or more compared with the 1-g SAR. The 1-g SAR is a more meaningfulmeasure of localized RF radiation absorption inside the head (where a wholeeye has a total mass of about 10g) and the trunk.

2.2 Current Compliance Evaluation Process

Remark. Current exposure guidelines are not appropriate for mmWave devices:

FCC regulations provide guidelines on how to perform the compliance evalu-ation process. Below 6 GHz and for devices at a distance from the human bodybelow 20 cm, SAR should be evaluated. If the devices are above 20 cm awayfrom the body (e.g.: base stations), PD evaluation should be performed. Above6 GHz, PD measuremes should always be performed. If the device is more than 5cm away from the body, measurements are enough otherwise, simulations usingnumerical modeling techniques (FDTD or FEM) will be required. This meansthat, at mmWaves, devices operating very close or in direct contact with thehuman body (less than 5 cm) the compliance evaluation with PD should be com-puted numerically. Since in the near-field, the computation will be made morecomplicated because it will depend on position, antenna geometry, orientationetc (Fig. 7).

Remark. Current SAR limits are not appropriate to be used at mmW frequen-cies:

A 60 GHz phased array with an assumed largest dimension of 10 mm (farfield at 4 cm), an antenna gain of 10 dBi and transmitting 100 mW will have apeak local (unaveraged) SAR level on the human skin of 22 W/kg if placed at

9

10 cm from the body. Nonetheless, the PD at this distance is only 0.8 mW/cm2,that is below the FCC allowed value of 1 mW/cm2. It seems, therefore, that noadverse thermal effect should take place.

Remark. Simulation and measurements of temperature and temperature in-crease in the near field of mmWave devices should be more valuable than esti-mates of PD alone:

In the example above, the distance considered is in the far-field. However,many mmWave portable devices would work on separation distances below 4cm and the tissue will be exposed to near field radiations. In this situation,numerical simulations can be used to evaluate the exposure to electromagneticenergy. These methods are well established at lower frequencies, but are not yetwidely established at mmWave frequencies.

2.3 Skin Properties and Heating

Remark. mmWave energy penetrates the stratum corneum easily but is rapidlyabsorbed within the deeper epidermis and dermis and does not propagate furtherinto the body:

Human skin consists of two primary layers: an outer epidermis (0.06 - 0.1mm thickness) and an underlying dermis (1.2 - 2.8 mm thickness); the surfacelayer of the epidermis is called stratum corneum (0.012 - 0.018 mm thickness).The stratum corneum has low water content (15-40%) and the total water con-centration in the rest of the epidermis and dermis is 70-80%. The high watercontent leads to high absorption coefficients at mmWave electromagnetic energyin the tissue.

Remark. Well-established permittivity database is missing for the millimeter-wave band. Therefore, further measurements of different body sites and differenthuman subjects are needed for developing accurate models:

some researchers have extrapolated complex permittivity of human skin atmmWave band from experimental data available at microwave frequencies; oth-ers have conducted direct measurements for characterization of the humanskin (Fig. 8 and 9).

Since mmWave wavelengths are small (1-10 mm) compared with the dimen-sions of the human body, a semi-infinite flat surface model of the human skin isreasonable and the reflection coefficients can be estimated based on this assump-tion. Using the skin models suggested above, the reflection coefficients at theair/skin interface at 60 GHz for parallel and perpendicular polarizations havebeen computed. The results show that 30-40% of incident power is reflectedat the skin surface. At 40 GHz, 43% of the incident power is reflected at theskin interface (normal incident plane wave), at 100 GHz, the power reflectioncoefficient decreases to 30% and more power is transmitted into the body athigher frequencies. The Brewster angles, where almost all energy for parallelpolarization is absorbed in the human skin, lie in the range of 60◦ to 80◦ atvarious frequencies (Fig. 10).

Remark. A single-layer or a multilayer skin model is sufficient to reliably eval-uate mmWave reflection and electromagnetic penetration in the skin:

10

Figure 8: Predicted skin relative permittivity according to model parameterspresented by several researchers from 10 to 100 GHz [7].

Figure 9: Predicted skin relative conductivity according to model parameterspresented by several researchers from 10 to 100 GHz. [7]

11

Figure 10: Power reflection coefficients at the air/skin interface at 60 GHz usingdifferent skin model parameters for (a) parallel and (b) perpendicular polariza-tions. Power reflection coefficients at the air/skin interface using Gabriel’s skinmodel at 40, 60, 80, and 100 GHz for (c) parallel and (d) perpendicular polar-izations [7].

the various skin models show that the penetration depth decreases rapidlywith the increase of frequency (Fig. 11); more than 90% of the transmittedelectromagnetic power is absorbed within the epidermis and dermis layers (Fig.12.

Remark. Accurate human body model are needed to predict the temperatureelevation for safety assessments due to mmW radiation:

Although the penetration is low, the heating of human tissue may extenddeeper than the epidermis and dermis layers including fat and muscles; bloodflow also helps to reduce the temperature elevation induced by mmWave radi-ation. A 1-D three-layer human tissue of skin, Subcutaneous Adipose Tissue(SAT) and muscle was modeled and the distribution of SARρ was calculatedfor a PD of 10 W/m2 (FCC and ICNIRP restrictions for the general public).Fig. 13 shows that the radiation power is confined to a shallower depth at ahigher frequency. Simulation results have shown that a one-layer skin modelgives different temperature results from a 3-layer skin model (Fig. 14).

To design appropriate antennas for these applications, the influence of thebody on the antenna performances should be carefully taken into account there-fore, an accurate characterization of skin dielectric properties is required.Besides, one of the promising currently unexplored solutions to reduce suscep-tibility to shadowing is to implement re-configurable millimeter-wave wearableantennas.

Attenuation of most garment materials is negligible: it never exceeded 3 dBfor frequencies below 350 GHz. Clothing in direct contact with the skin mightact as an impedance transformer, resulting in the enhancement of power trans-

12

Figure 11: The penetration depth in the human skin with the increase of expo-sure frequency [7].

Figure 12: The attenuation of SAR in the skin for an incident PD of 10 W/m2

at 60 GHz [7].

13

Figure 13: The SARρ distribution due to 10 W/m2 mmWave radiation at 40,60, 80 and 100 GHz [7].

Figure 14: The steady-state temperature elevation at incident power densities of10 and 50 W/m2 due to 60 GHz electromagnetic wave exposure for a three-layermodel and a one-layer model that consists of skin only [7].

14

mission into the body [8]. An air gap of up to 2 mm between the clothes andthe skin decreases the body transmission. For on-body applications, the charac-terization of the propagation channel is of utmost importance as it significantlydiffers from the free-space one, and no information is currently available in thescientific literature on this issue.

2.4 Other mmWave Effects

There are several references in the literature that suggest other positive or nega-tive effects of mmWaves on the human body. Most of these studies need furtherexperiments:

• eyes are particularly vulnerable to mmWave radiation-induced heating asthey lack sufficient blood flow to redistribute generated heat;

• mmWave are not genotoxic; this is probably due to the nonionizing na-ture of mmWave band. Therefore, mmWave radiation should not inducecancer;

• mmWaves could activate natural kill (NK) cells that remove tumor cells;

• mmWaves might accelerate healing of wounds and heal wounded skin with-out leaving scars. Several beauty clinics in the former Soviet Union haveused mmWave therapy in cosmetology.

Among the potentially critical aspects there are:

• membranes might be affected by mmWaves. PD levels close to thosetypically expected from wireless communication systems (0.9 mW/cm2)can induce structural modifications;

• long-term effects of heating due to mmWave frequencies are unknown.

3 Antennas for 5G Wearable Applications

Several wearable antennas for 5G applications have been suggested in the re-search literature. It has been shown that, the impact of the human body onthe antenna characteristics can be almost negligible and that even high inputpowers (550 mW) lead to exposure levels below international exposure limits.A more exhaustive description of mmWave antennas for on- and off-body com-munications can be found on the attached slides. Suggested readings aboutwearable antenna are: [9, 10, 11, 12, 13, 14, 15, 16].

• On/off-body communication, endfire antenna (Fig. 15) is described in [14].Max on-body gain 15.2 dBi at 57-64 GHz band on RT Duroid 5880 sub-strate, antenna/body spacing of 5.6 mm. The data presented demonstratethat, when the antenna is mounted on a phantom and/or bent, its per-formances remain satisfactory in terms of reflection coefficient, radiationpattern and efficiency.

15

Figure 15: Endfire Yagi-Uda mmWave antenna [14].

Figure 16: Enfire Yagi Uda mmWave antenna on textile substrate [16].

• On-body communication, endfire antenna (Fig. 16) on textile substrateis studied in [16]. Max on-body gain 11.9 dBi at 57-64 GHz band. Theantenna consists of a microstrip-fed Yagi-Uda with a driven dipole and10 directors printed on top layer of 0.2 mm-thick cotton fabric. The mi-crostrip truncated ground plane is located at 1 mm from the the drivingdipole and acts as a reflector. The driving dipole and the balun betweenthe microstrip feed and dipole are built on the top and bottom sides ofthe textile substrate. The S11 is affected by antenna/body separation at5 mm and 1 mm. The radiation pattern changes with d; at 0 mm, thedipole is short-circuited. The short circuit can be avoided by embeddingthe antenna between two fabrics.

• Off-body communication with mmWave patch antenna array is presentedin [15]. A max on-body gain 8 dBi array at 57-64 GHz band is reported(Fig. 17), with a max distance from the receiver of 5.4 m and 1.6 m un-

16

Figure 17: Off-body communication mmWave patch array [15].

der LoS and NLoS respectively (PT = 10 dBm, GRX = 15 dBi, receiversensitivity = -50 dBm.). The 2x2 patch antenna array was fabricated onsubstrate of commercial textile. The fabrication process consists in us-ing a thin and flexible 0.07 mm thick copper foil with ground plane onShieldIt Super. The copper foil is etched with a laser machine. The rela-tive permittivity of the substrate was retrieved using open-stub technique.Differently from on-body communications, the human body effects on theantenna characteristics are very weak thanks to the ground plane. Bend-ing effects on s11 have been measured and crumpling effects have beensimulated. In both cases, the s11 remains well matched in the band of in-terest. The authors suggest that non-textile antennas should be employedwhen possible since they have higher efficiency.

4 Practical Issues

Research literature on 5G provides many additional information that the readercan investigate:

• mmWave antenna on textile fabrication techniques are described in [15];

• mmWave wearable antennas feeding techniques are studied in [17];

• a mmWave phantom for testing mmWave antennas on skin is describe in[18];

• an early testbed for future mmWave academic prototyping and demon-stration is described in [2].

17

5 Key Labs Involved in mmWave Studies

• Theodore S. Rappaport, Ph.D.Web: nyuwireless.org, email: theodore.rappaport@nyulangone.orgFounder and center director of NYU WIRELESS: NYU’s Langone Medi-cal Center, NYU’s Courant Institute, and NYU-Poly’s electrical and com-puter engineering department;

• Robert W. Heath Jr., Ph.D.Web: profheath.org, email: rheath@utexas.eduThe Cullen Trust for Higher Education Professorship in Engineering. Wire-less Networking and Communications Group Department of Electrical andComputer Engineering. University of Texas at Austin;

• Maxim Zhadobov, Ph.D.Web: zhadobov.fr, mail: maxim.zhadobov@ieee.orgLeader of Biomedical Electromagnetics Group and Head of WAVES Teamof IETR. Institute of Electronics and Telecommunications of Rennes (IETR)/ French National Center for Scientific Research (CNRS).

References

[1] J. G. Andrews, S. Buzzi, W. Choi, S. V. Hanly, A. Lozano, A. C. K. Soong,and J. C. Zhang, “What will 5g be?” IEEE Journal on Selected Areas inCommunications, vol. 32, no. 6, pp. 1065–1082, June 2014.

[2] A. A. R. W. H. J. Cody Scarborough, Kiran Venugopal, “Beamform-ing in millimeter wave systems: Prototyping and measurement results,”arXiv.org, Aug 2018.

[3] M. Matthaiou, G. K. Karagiannidis, E. G. Larsson, T. L. Marzetta, andR. Schober, “Guest editorial: Large-scale multiple antenna wireless sys-tems,” IEEE Journal on Selected Areas in Communications, vol. 31, no. 2,pp. 113–116, February 2013.

[4] F. R. Farrokhi, A. Lozano, G. J. Foschini, and R. A. Valenzuela, “Spectralefficiency of fdma/tdma wireless systems with transmit and receive antennaarrays,” IEEE Transactions on Wireless Communications, vol. 1, no. 4, pp.591–599, Oct 2002.

[5] T. L. Marzetta, “Noncooperative cellular wireless with unlimited numbersof base station antennas,” IEEE Transactions on Wireless Communica-tions, vol. 9, no. 11, pp. 3590–3600, November 2010.

[6] R. W. Heath, N. Gonzalez-Prelcic, S. Rangan, W. Roh, and A. M. Sayeed,“An overview of signal processing techniques for millimeter wave mimosystems,” IEEE Journal of Selected Topics in Signal Processing, vol. 10,no. 3, pp. 436–453, April 2016.

[7] T. Wu, T. S. Rappaport, and C. M. Collins, “Safe for generations to come:Considerations of safety for millimeter waves in wireless communications,”IEEE Microwave Magazine, vol. 16, no. 2, pp. 65–84, March 2015.

18

[8] M. Zhadobov, N. Chahat, R. Sauleau, C. LeQuement, and Y. LeDrean,“Millimeter-wave interactions with the human body: State of knowledgeand recent advances,” International Journal of Microwave and WirelessTechnologies, vol. 3, no. 2, pp. 237 – 247, April 2011.

[9] N. Chahat, M. Zhadobov, R. Sauleau, and K. Ito, “A compact uwb antennafor on-body applications,” IEEE Transactions on Antennas and Propaga-tion, vol. 59, no. 4, pp. 1123–1131, April 2011.

[10] A. T. Alreshaid, O. Hammi, M. S. Sharawi, and K. Sarabandi, “A mil-limeter wave switched beam planar antenna array,” in 2015 IEEE Inter-national Symposium on Antennas and Propagation USNC/URSI NationalRadio Science Meeting, July 2015, pp. 2117–2118.

[11] X. Lin, B. Seet, and F. Joseph, “Fabric antenna with body temperaturesensing for ban applications over 5g wireless systems,” in 2015 9th Interna-tional Conference on Sensing Technology (ICST), Dec 2015, pp. 591–595.

[12] S. F. Jilani, B. Greinke, Y. Hao, and A. Alomainy, “Flexible millimetre-wave frequency reconfigurable antenna for wearable applications in 5g net-works,” in 2016 URSI International Symposium on Electromagnetic Theory(EMTS), Aug 2016, pp. 846–848.

[13] J. Puskely, M. Pokorny, J. Lacik, and Z. Raida, “Wearable disc-like antennafor body-centric communications at 61 ghz,” IEEE Antennas and WirelessPropagation Letters, vol. 14, pp. 1490–1493, 2015.

[14] G. A, N. Chahat, C. Leduc, M. Zhadobov, and R. Sauleau, “End-fire an-tenna for ban at 60 ghz: Impact of bending, on-body performances, andstudy of an on to off-body scenario,” electronics, vol. 3, pp. 221–233, April2014.

[15] N. Chahat, M. Zhadobov, S. A. Muhammad, L. L. Coq, and R. Sauleau,“60-ghz textile antenna array for body-centric communications,” IEEETransactions on Antennas and Propagation, vol. 61, no. 4, pp. 1816–1824,April 2013.

[16] N. Chahat, M. Zhadobov, L. L. Coq, and R. Sauleau, “Wearable endfiretextile antenna for on-body communications at 60 ghz,” IEEE Antennasand Wireless Propagation Letters, vol. 11, pp. 799–802, 2012.

[17] C. Leduc and M. Zhadobov, “Impact of antenna topology and feeding tech-nique on coupling with human body: Application to 60-ghz antenna ar-rays,” IEEE Transactions on Antennas and Propagation, vol. 65, no. 12,pp. 6779–6787, Dec 2017.

[18] N. Chahat, M. Zhadobov, and R. Sauleau, “Broadband tissue-equivalentphantom for ban applications at millimeter waves,” IEEE Transactions onMicrowave Theory and Techniques, vol. 60, no. 7, pp. 2259–2266, July 2012.

19