Exploration of antenna and passive beamforming techniques for wireless energy harvesting and transfer

HAL Id: tel-02905411

https://tel.archives-ouvertes.fr/tel-02905411

Submitted on 23 Jul 2020

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de

Exploration of antenna and passive beamforming techniques for wireless energy harvesting and transfer

Erika Vandelle

To cite this version:

Erika Vandelle. Exploration of antenna and passive beamforming techniques for wireless energy harvesting and transfer. Optics / Photonic. Université Grenoble Alpes, 2019. English. �NNT : 2019GREAT060�. �tel-02905411�

!

!"#$%&

!"#$%"&'()*$%+(%,$-.(%.(%

!"#$%&'(!%()*(#"++&,*&$%(&,-.%'/-$%(

0'%,"1)%(*)2%/(

/012*-+*'1%3%!"#$%&'('#()*+$,-./%&'01'2(

!""#$%&'()(*$%"(+,&-&./&'0(&.123&

&

!"#$%&'#%()*"(

%'()*&+,-.%//%&

!

4567(%.*$*,1(%0-$%3*0456&(78!9:;(5.,-'22'&.;(:.'0,<='(>958%

('%2".*$*,1(%0-$%?'(@8;(5.,-'22'&.;(A1,='(5,=B#'160$%&'(+'(

C,0#./*=;(D$E,0(FAC!8);(G,.+'*&H(>95;(:&2#*I,(J)K>LJ(

)!:)>:8AM;(80$I'.2$#/(:.'0,<='(J="'2%

%

0$10-$1(%-#%7(*)%.#%L*<,.*#,$.'(>CA54LJFN%

.-)7%=OP1,='(K,1#,.*='(P='1#.,0$%&';(P='1#.,#'160$%&';(

J&#,E*#$%&'('#(3.*$#'E'0#(+&(D$Q0*=(RAAJ3DS(

!

!"#$%&'()%*+,-+.%$/()%*.+

'*(-**')&-.+-(+,-+0%&1'()%*+

#'..)2-+,-+0').3-'/"+#%/&+$'+

&43/#4&'()%*+-(+$-+(&'*.0-&(+

,54*-&6)-+.'*.+0)$+

(

4567(%7"#'()#(%0#&+*9#(:()'%+((T(UV(K/1'E<.'(WXUYZ8(

.(;-)'%+(%<#$=%2":0"71%.(%3%%

!"#$%&'()**#%+#,'-.%

!"+,%$$%-"%.(/+"0%*-1(23!.(!"#$40%&'%(

!($%/0#12*3(-%4/5/&6%

5*6'"%(0%(7+&,#"%&8%$.(9&4:%"$4'#(!*-;(<*=*'4%".(>*))+"'%-"(

!($%728)#*%7#((#('%

!"+,%$$%-".(9&4:%"$4'#(7?'%(0@AB-".(>*))+"'%-"(

!($%9""2*-#0%+(#)*2%

C+8'%-".(<8D&%40%"(E;%8'"48.(E1*F4&*'%-"(

!"#$%4:)%;-<*:%=2*%>?/@A%

C+8'%-"%.(GD*;H$.(2&:4'#%(

!($%42*BC:-%=D?@A%

!"+,%$$%-".(I"%&+=;%(23!.(5%F="%(

!($%5#%ED%

!"+,%$$%-".(J8+;%(!+;K'%8D&4L-%(0%(5+&'"#*;.(5%F="%(

!($%6)"'*%>9!?DF%

5*6'"%(0%(7+&,#"%&8%$.(9&4:%"$4'#(0%(/+"0%*-1.(5%F="%(

!

Remerciements

Je tiens tout d’abord à remercier Madame Corinne Desjoux, Professeure de l’Institut Polytechnique de Bordeaux, d’avoir accepté de présider mon jury de thèse. J’adresse également ma reconnaissance à Monsieur Alexandru Takacs, Maître de Conférences de l’Université Paul Sabatier, et Monsieur Fabien Ferrero, Professeur de l’Université Côte d’Azur, d’avoir rapporté ces travaux de thèse. Merci également à Madame Thi Quynh Vân Hoang, Docteure de Thalès et Monsieur Emmanuel Dreina, Docteur de Schneider Electric pour l’intérêt porté à mes travaux et d’avoir accepté d’être membre de mon jury de thèse.

J’adresse tous mes remerciements à mon directeur de thèse, Monsieur Tan-Phu Vuong, Professeur de l’Institut Polytechnique de Grenoble, pour toute la confiance qu’il m’a adressée depuis mon stage de Master 2. Merci pour tous tes conseils, ton soutien, ta bienveillance et ta gentillesse. Je remercie aussi tout particulièrement mon co-directeur de thèse, Monsieur Ke Wu, Professeur de l’Ecole Polytechnique de Montréal, pour tous ses précieux conseils, ses corrections et sa vision de la recherche très inspirante. Merci aussi de m’avoir permis de passer six mois au sein de Poly-Grames à Montréal, ce fût un séjour très enrichissant tant professionnellement que personnellement, remplis d’apprentissage et de rencontres.

Je remercie très chaleureusement mon co-encadrant Monsieur Simon Hemour, Maître de Conférences à l’Université de Bordeaux pour son précieux encadrement. Merci pour tes conseils, ton aide, tes idées et les opportunités qui m’ont été offertes grâce à toi. Merci aussi pour ta motivation et ta passion qui ont été très souvent contagieuses. J’adresse aussi toute ma gratitude à Monsieur Gustavo Ardila Rodriguez, Maître de Conférences de l’Université Grenoble Alpes, pour toute sa bienveillance et sa gentillesse très encourageantes. Tu as toujours fait preuve d’une grande disponibilité, de réactivité et de soutien, merci beaucoup pour ça.

J’adresse mes sincères remerciements à Monsieur Nicolas Corrao, responsable de la plate-forme de caractérisation hyperfréquences pour toute l’aide et le temps qu’il m’a consacré, que ce soit pour la réalisation des circuits et les mesures. Merci pour ta confiance et ta gentillesse. Je tiens également à remercier Monsieur Antoine Pisa, technicien à Phelma, pour tous les circuits qu’il a fabriqués, toujours avec le sourire. Je souhaite remercier également les techniciens de Poly- Grames Jules Gauthier, Traian Antonescu et Maxime Thibault pour la fabrication de mes circuits et l’aide apportée pour les mesures que j’ai pu faire au cours de mon séjour.

Je tiens à remercier mes précieux collègues de l’IMEP-LAHC et d’ailleurs, pour leur aide, leur

soutien et leur amitié. J’ai une pensée particulière pour ceux qui, depuis le Master, m’ont aidée,

inspirée et encouragée : Clément Pornin, Edouard Rochefeuille, Mohammed Réda Bekkar et

Chhayarith Heng Uy. Merci à tous les autres membres de l’IMEP-LAHC et de Poly-Grames qui ont rendu agréable le quotidien de mes trois années de thèse.

Enfin, je tiens à remercier ma mère, mon père, Xavier et mes amis qui m’ont encouragée durant ces trois années.

!

Content

List of Figures ... 5

List of Tables ... 13

List of Acronyms and Abbreviations ... 14

1 General Introduction ... 17

1.1 General Context: The Need for Battery-less Sensors and Passive Sensing Applications ... 18

1.1.1 The “Wireless Trends” ... 18

1.1.2 Wireless Power Transfer (WPT) and Wireless Energy Harvesting (WEH) Definitions ... 19

1.1.3 Towards Battery-less Sensors and Passive Sensing Applications ... 23

1.2 Wireless Energy Harvesting (WEH) ... 25

1.2.1 Ambient Power Densities ... 25

1.2.2 Power Regulations ... 26

1.2.3 Rectenna ... 28

1.2.4 Rectifier Architectures ... 29

1.3 Low-Power Rectenna Design: Challenges ... 30

1.3.1 Rectenna Sensitivity ... 30

1.3.2 Rectifier Sensitivity ... 31

1.3.3 Activation Distance ... 33

1.3.4 Rectenna Efficiency ... 34

1.3.5 Ambient Signal Diversity ... 35

1.4 Low-Power Rectenna Design: State-of-the-Art ... 36

1.4.1 Multi-source Rectenna to Address Rectifier’s Low-Efficiency ... 37

1.4.2 Rectenna Array and Frequency Selective Surface to Address Low-Power Operation ... 38

1.4.3 RF Power Combination Techniques to Address Low-Power Operation ... 39

1.4.4 Multi-frequency Rectenna to Address Frequency Diversity ... 40

1.4.5 Multi-polarization Rectenna to Address Polarization Diversity ... 41

1.4.6 Multi-directional Rectenna to Address Spatial Diversity ... 41

1.4.7 FoM and Operation Indicator of RF Energy Harvesters ... 42

1.5 Contributions and Organization of the Manuscript ... 44

1.5.1 Contributions of the Work Disclosed in this Thesis ... 44

1.5.2 Organization of the Manuscript ... 45

2 Low Power Densities Collection and RF-to-DC Conversion ... 47

2.1 Overall View ... 48

2.2 Rectification ... 48

2.2.1 Non-linear Behavior of Diodes ... 49

2.2.2 Rectification Efficiency ... 51

2.2.3 Input Impedance of Diodes ... 55

2.3 Impedance Matching ... 59

2.3.1 The Bode-Fano Criterion ... 59

2.3.2 Matching Network Efficiency ... 60

2.4 Captured RF Power ... 63

2.4.1 Output RF Power of an Antenna ... 63

2.4.2 Output RF Power of an Antenna Array ... 64

2.5 Delivered DC Power ... 68

2.5.1 Output DC Power of a Rectenna ... 68

2.5.2 Output DC Power of a Rectenna Array ... 69

2.5.3 Output DC Power of a Rectifying Antenna Array ... 70

2.5.4 Evaluation of the DC Power Delivered by Rectennas ... 72

2.6 The Probability of Collecting Energy in Ambient Scenarios ... 73

2.6.1 Frequency Spectrum ... 74

2.6.2 Polarization ... 74

2.6.3 Spatial Coverage ... 77

2.6.4 A Figure-of-Merit for Rectennas Dedicated to Ambient Scenarios ... 80

2.7 Conclusions of Chapter 2 ... 83

3 Antenna and Rectenna Elements ... 84

3.1 Microstrip Antennas with Enhanced Aperture Efficiency ... 85

3.1.1 Microstrip Antenna with Enhanced Directivity and Bi-directional Pattern ... 85

3.1.2 Microstrip Antenna with Enhanced Radiation Efficiency ... 90

3.1.3 Miniaturized Microstrip Antennas an Air-Filled Paper Substrate ... 94

3.1.4 Comparison of the Different Antennas ... 97

3.2 Rectenna in a 50

System ... 98

3.2.1 Single Series Diode Rectifier ... 98

3.2.2 Rectifier Results ... 99

3.2.3 Rectenna Measurement ... 102

3.2.4 Comparison of the 50- ! Rectennas ... 103

3.3 Multiband Conjugate-Matched Rectenna ... 104

3.3.1 Voltage Doubler Rectifier ... 104

3.3.2 Conjugate-Matched Antenna ... 106

3.3.3 Rectenna Results ... 108

3.4 Performance Comparison of the Proposed Rectennas ... 111

3.5 Conclusion of Chapter 3 ... 113

4 Efficient Ambient Energy Harvesting System with Optimal Angular Coverage 115 4.1 General Principle ... 116

4.1.1 Beam-Forming Networks in Energy Harvesting: Analytical Approach ... 116

4.1.2 Proposed System ... 117

4.1.3 RF Combination in the Proposed Rectenna ... 118

4.2 Condition of Efficiency of the Proposed Rectenna ... 119

4.2.2 Harvesting Capability of the Proposed Rectenna ... 123

4.2.3 Butler Matrix Scalability ... 124

4.3 Design of Passive Power RF Combining Circuits ... 125

4.3.1 Hybrid Couplers and Crossovers ... 126

4.3.2 Miniaturized 4x4 Butler Matrices ... 132

4.4 Design of the Antennas and Rectifiers ... 137

4.4.1 Multidirectional Antenna Arrays ... 137

4.4.2 Electric Dipole Antenna ... 140

4.4.3 Rectifier ... 141

4.5 Rectenna Measurement ... 143

4.5.1 Efficiency and DC Power Patterns ... 143

4.5.2 Sensitivity ... 145

4.5.3 Performance Comparison ... 148

4.6 Conclusion of Chapter 4 ... 150

5 Scalable and Reconfigurable Energy Harvesting System ... 152

5.1 General Principle ... 153

5.1.1 DC Combinations of Rectifiers’ Outputs: Analytical Approach ... 153

5.1.2 Proposed Rectenna Unit-Cell (RU) ... 154

5.1.3 Reconfigurability of the System ... 158

5.1.4 Condition of Dominance of the Two Reconfigurable States ... 159

5.2 Design of the Rectenna Unit Cells (RUs) ... 162

5.2.1 Rectifier ... 162

5.2.2 Rectifying Unit Cell ... 164

5.2.3 Multiple Rectifying Network Unit Cells (RNUs) ... 166

5.2.4 Patch Antenna ... 169

5.3 Power and Voltage Gain Factors ... 170

5.3.1 DC Combination Efficiency ... 170

5.3.2 Scenario A: Power and Voltage Gain Factors of 2-RNUs in Direction of Maximum Intensity ... 172

5.3.3 Scenario A: Power and Voltage Gain Factors of 4-RNUs in Direction of Maximum Intensity ... 174

5.3.4 Scenario B: Power and Voltage Gain Factors of 2 RUs in Direction of Maximum Intensity ... 176

5.4 Example of a Multi-Function System ... 177

5.4.1 Energy Harvesting Function ... 177

5.4.2 Passive Localization Function ... 178

5.5 Conclusion of Chapter 5 ... 180

6 General Conclusion and Outlook ... 182

Annexes ... 186

A.1 Losses in Microstrip Lines ... 186

A.2 Array Factor Matlab Codes ... 192

A.3 DC Connection of Rectifier’s Outputs ... 196

A.4 Material RF Characterization ... 200

Bibliography ... 207

List of Publications ... 217

Abstract ... 219

Résumé ... 220

List of Figures

Figure 1-1 2018 Gartner hype curve for emerging technologies [4]. ... 18 Figure 1-2 Non-radiative wireless power transfer in the near-field region with resonant inductive coupling. ... 21 Figure 1-3 Radiative wireless power transfer in the far-field region. ... 21 Figure 1-4 Block diagram of the definitions of Wireless Power Transfer (WPT) and Wireless Energy Harvesting (WEH). ... 22 Figure 1-5 Smart city equipped with IOT wireless sensors. ... 23 Figure 1-6 Wirelessly powered sensors communicating with a base station in (a) a SWIPT and (b) a WPCN scenarios. ... 23 Figure 1-7 Wireless power supply of (a) low-duty cycle and low-power sensors and (b) backscattering sensors. ... 24 Figure 1-8 The two scenarios of RF energy harvesting: (a) the energy is harvested from a RF source that intentionally transmits power to the harvester (b) harvesting ambient RF power present in the environment. ... 25 Figure 1-9 Photograph of the first rectenna (1963) made up of half-wave dipoles each terminated with full bridge rectifiers [36]. ... 28 Figure 1-10 Rectenna composition for low-power operation. ... 29 Figure 1-11 Rectifier topologies for WEH: (a) single series diode, single shunt diode, voltage doubler and modified Greinacher rectifiers. ... 29 Figure 1-12 Harvestable power density (nW.cm

) given as a function of the RF power (dBm) delivered to the rectifier and of the antenna gain (dBi). ... 31 Figure 1-13 State-of-the-art of rectifiers’ efficiencies (%) (with matching network) at the minimum input power (dBm) that leads to an output DC voltage of 165 mV with the optimal load (theoretically corresponding to a 330-mV open-circuit output voltage). ... 32 Figure 1-14 Power density and RF power received by the rectenna as a function of the distance range between the rectenna and the emitting source, at 868 MHz. ... 34 Figure 1-15 State-of-the-art of (a) the relation between the delivered DC power density ( " W.cm

2

) of a rectenna with the received RF power density ( " W.cm

): global efficiency; and (b) the

minimum collected DC power entering the boost converter. ... 35

Figure 1-16 Ambient signal diversity. ... 36

Figure 1-17. Rectification efficiency of multi-source rectennas: (a) kinetic+RF sources [69] (b)

multi-tone RF sources [49]. ... 37

Figure 1-18. Photographs of rectenna arrays (a) [73] (b)[75], and frequency-selective surface

rectennas (c) [79] (d) [81]. ... 38

Figure 1-19 Radiation patterns and photographs of rectennas made up of antenna arrays: single

port (a) [49] and multiple ports (b) [82] (c) [83]. ... 39

Figure 1-20 Multi-branch rectifiers (a) [56] and (b) [64] and rectification efficiency of multi-

frequency rectennas: (c) [56], (d) [64]. ... 40

Figure 1-21. (a) Photograph of a RF energy harvester for autonomous UWB tag with 4- polarization capabilities [85] and its (b) activation distance’s improvement. ... 41

Figure 1-22. (a) Photographs of multi-directional rectennas: (a) [92], (b) [90], (c) [91]. ... 42

Figure 1-23 Rectenna Figure-of-Merit (RFoM) [93]. ... 43

Figure 1-24 Rectenna Topology Indicator (RTI) [58] ... 44

Figure 2-1 Efficiency diagram of the rectenna. ... 48

Figure 2-2 Shockley diode model with parasitic elements. ... 49

Figure 2-3 I-V Characteristics of commercial Schottky diodes. ... 50

Figure 2-4 Schematic of the simulated (a) single series and (b) voltage doubler rectifiers ... 53

Figure 2-5 Junction resistance ( # ) of Schottky Diodes. ... 53

Figure 2-6 (a) Rectification efficiency (%) (with optimal load) and (b) open-circuit voltage (mV) of different Schottky diodes mounted in the single series topology with a lossless L-matching network. f = 2.45 GHz. ... 54

Figure 2-7 (a) Rectification efficiency (%) (with optimal load) and (b) open-circuit voltage (mV) for 2 Schottky diodes in both the single series and the voltage doubler topologies with a lossless matching network. f = 2.45 GHz. ... 55

Figure 2-8 Rectification efficiency (%) of the SMS7630-061L diode with a L matching network optimized for different input powers. f = 2.45 GHz. ... 56

Figure 2-9 (a) Equivalent circuits of the SMS7630 and SMS7621 Schottky diodes in the (a) SC- 79 and (b) SOT-23 packages. (c) Measurement setup using a TRL-calibration kit (d) mounted in a test fixture. ... 57

Figure 2-10 Input (a) resistance and (b) reactance of the Schottky diodes SMS7630 and SMS7621 in the SOT-23 and SC79 packages for an input power of -20 dBm. ... 58

Figure 2-11 Input (a) resistance and (b) reactance of the Schottky diodes SMS7630 and SMS7621 in the SOT-23 and SC79 packages for an input power of -10 dBm. ... 58

Figure 2-12 RC load. ... 59

Figure 2-13 Theoretical reflection coefficient of an ideal single serial diode rectifier composed of the Schottky diode SMS7630-061L with an output capacitor $% = 10 nF. ... 60

Figure 2-14 Theoretical matching efficiency as a function of the power match factor m for different net quality factors and different numbers of L matching networks cascaded. ... 62

Figure 2-15 Array geometry: (a) linear and (b) planar array. ... 65

Figure 2-16 Scan efficiency. ... 68

Figure 2-17 Single rectenna. ... 68

Figure 2-18 Rectenna array configuration. ... 69

Figure 2-19 Rectifying antenna array collecting energy (a) in the broadside direction and (b) away from the broadside direction. ... 71

Figure 2-20 Environment A (a), environment B (b) and environment C (c). ... 72

Figure 2-21 Radiation pattern of (a) an unidirectional antenna and (b) an omnidirectional antenna. ... 78 Figure 2-22 Top: probability that the power received at the angle &'(is higher than a portion

of a reference power )*+,((-. /01( as a function of the directivity (dBi) with omnidirectional and unidirectional radiation patterns of the forms 23(453'& . 6'33& and 23(73' . (8463&

respectively with 24 . 9 . Bottom: Solid angle normalized with respect to that of a complete sphere as a function of the directivity (dBi). ... 79 Figure 2-23 Harvesting capability (

%.sr) (2.71) of state-of-the-art rectennas at the minimum power density ( " W.cm

) resulting in an output DC voltage of 165 mV across the optimal load.

... 81 Figure 2-24 Output DC power ( " W) in ambient scenarios of state-of-the-art rectennas at the minimum power density ( " W.cm

) resulting in an output DC voltage of 165 mV across the optimal load. ... 82 Figure 3-1 (a) Distribution of the electric field in the TM01 mode and (b) equivalent magnetic current densities of the fringing field of a microstrip antenna [103]. ... 86 Figure 3-2 Electric field distribution and equivalent magnetic current in a slot. ... 86 Figure 3-3 Antenna geometry. ... 87 Figure 3-4 Electric field in the yoz-plane of the microstrip antenna (a) without the slot (b) and with the slot at 2.45 GHz. ... 88 Figure 3-5 3D radiation pattern of the antenna when d increases (f=2.45 GHz). ... 88 Figure 3-6 (a) Aperture efficiency (%) and (b) radiation efficiency (%) as a function of the distance d (mm) for various :4 at 2.45 GHz. ... 89 Figure 3-7 Reflection coefficient (dB) and (b) gain pattern (dBi) in the E- and H-planes at 2.45 GHz of the antenna. ... 90 Figure 3-8 Microstrip antenna on paper substrate [112]. ... 91 Figure 3-9 Evolution of the directivity (dBi) and radiation efficiency (%) with the air thickness.

... 93

Figure 3-10 (a) Reflection coefficient (dB). ... 93

Figure 3-11 Geometry of (a) the antenna; (b) the different layers and (c) the parameters of each

layer of the antenna; (d) geometry of the antenna when flattened. ... 94

Figure 3-12 (a) Surface current on the antenna’s ground plane and distribution of the electric

field in the (b) XoZ and (c) YoZ planes at 2.45 GHz. ... 95

Figure 3-13 Evolution of (a) reflection coefficient (dB) and of (b) directivity (dBi) of the antenna

with the slot width W. ... 95

Figure 3-14 Fabricated antenna prototypes (a) folded and (b) unfolded. ... 96

Figure 3-15 (a) Reflection coefficient (dB) of the antenna prototypes. ... 96

Figure 3-16 Simulated and measured realized gain pattern (dBi) of the antennas in the E-plane

and H-plane (a) with copper tape and with (b) silver ink. ... 97

Figure 3-17 Layout of the rectifier (dimensions in mm). ... 98

Figure 3-18 Fabricated 50 # -rectifier. ... 99

Figure 3-19 Reflection coefficient (dB) (a) in simulation and measurement of the rectifier for an input power of -25 dBm. (b) Measured reflection coefficient (dB) of the rectifier for input powers of -25, -20 and -25 dBm. ... 100 Figure 3-20 Simulated and measured (a) DC voltage (mV) and (b) rectification efficiency (%) across a load of 5.1 k # at 2.45 GHz. ... 100 Figure 3-21 Rectification efficiency (%) of the rectifier as a function of the available power density ( " W.cm

) and the effective aperture of an antenna (cm

). ... 101 Figure 3-22 Measurement setup of the 50- # rectenna. ... 102 Figure 3-23 (a) Measured rectification efficiency (%) as a function of the available power density

" W.cm

of the rectenna. (b) Rectenna in the anechoic chamber. ... 103 Figure 3-24 Input impedance of the rectifier (a) without and (b) a series inductor of 8.2 nH for an input power of -10 dBm. (c) Typical impedance of a microstrip antenna. ... 104 Figure 3-25 (a) Measurement setup of the input impedance of the rectifier circuits: (b) without inductor and with (c) input inductor. ... 105 Figure 3-26 Input resistance and reactance -#1 of the rectifier for an input power of -10 dBm rectifier (a) with no series inductor and (b) with two series inductors of 5.6 and 8.2 nH. ... 106 Figure 3-27 Geometry of (a) the rectenna with (b) the parameters of each layer and (c) geometry of the rectenna when flattened. ... 106 Figure 3-28 Effect of the air gap: (a) increase of the radiation efficiency and (b) widening of the resonance bandwidth. ... 107 Figure 3-29 Evolution of the (a) impedance of the antenna ( # ) and of the (b) gain (dBi) with the parameters s and b. ... 108 Figure 3-30 Evolution of the (a) impedance of the antenna ( # ) and of the (b) gain (dBi) and front-to-back ratio (dB) with the width of the slot W. ... 108 Figure 3-31 (a) Rectification efficiency (%) of the (b) rectenna prototype (35 mm x 35 mm x 10 mm) at the two frequency bands 1.8 and 2.4 GHz as a function of the load (k #1 for various powers of -10 and -20 dBm at the rectifier’s input. ... 109 Figure 3-32 Measurement setup of the conjugate-matched rectenna. ... 109 Figure 3-33 The simulated and measured output DC voltage (mV) across a load of 6.2 k

of the rectenna (a) as a function of the frequency (GHz) at 1 and 10 " W/cm

, (b) as a function of the RF power density ( " W/cm

) at 1.8 and 2.45 GHz. ... 110 Figure 3-34 Simulated and measured (a) output voltage (mV) and (b) conversion efficiency (%) of the rectenna as a function of the RF power density ( " W/cm2) at 1.8 and 2.45 GHz. RL = 6.2 k # . ... 111 Figure 3-35 (a) Relation between ;<$ ( " W.cm

) and ;=> ( " W.cm

) and (b) between ;<$ ?

@5AB( ( " W.cm

) and ;=> ( " W.cm

) of state-of-the-art and proposed rectennas, at the

minimum power density ( " W.cm

) resulting in an output DC voltage of 165 mV across the

optimal load. The parasitic efficiency and the sensitivity are normalized to that obtained

at 2.45 GHz. ... 113

Figure 4-1 Directional radiation pattern with solid angle

of a (a) single antenna element, (b) 2 antennas aligned along the y-axis with a two port beam-forming network and (c) 2x2 antennas aligned along the x- and y- axis with a 4-port beam-forming network. ... 116 Figure 4-2 (a) Example of application of the rectenna. (b) Sketch of the rectenna. ... 118 Figure 4-3 Working principle of the BNF-based rectenna: RF power paths for (a) i=3, (b) i =4, (c) i=1 and (d) i=2. ... 119 Figure 4-4 CST Microwave Studio simulation in the schematic view. ... 120 Figure 4-5 Calculated and simulated radiation pattern of one antenna array associated to the Butler matrix compared to that of a single patch antenna and an electric dipole antenna (a) in the the E-plane and (b) H-plane, with 2 dB insertion losses ( C = -8 dB, DE>F =0.63) and 0 dB insertion losses ( C = -6 dB, DE>F =1). ... 121 Figure 4-6 Squared aggregate array factor (normalized) of the proposed system in the E-plane of the antenna arrays. ... 122 Figure 4-7 (a) Components of a standard 4x4 Butler matrix and (b) magnitude (in red) and phase difference (in green) at the output of the components. ... 126 Figure 4-8 Hybrid coupler. ... 126 Figure 4-9 Design steps of the “stubs” hybrid coupler: (a) transformation of the branch/through lines into “3-stubs" lines, (b) transformation of a stub into one two-step stub. ... 127 Figure 4-10 (a) Dimensions and (b) layout of the “stubs” hybrid coupler. ... 128 Figure 4-11 Simulated results of the “stubs” hybrid coupler in (a) magnitude and (b) phase.

... 129 Figure 4-12 (a) Hybrid coupler’s unit cell and (b) photograph of the fabricated coupler. ... 130 Figure 4-13 Simulated and measured results of the “lumped-elements” hybrid coupler in (a) magnitude and (b) phase. ... 131 Figure 4-14 Simulation results for the crossover in (a) magnitude and (b) phase. ... 132 Figure 4-15 (a) Layouts and (b) photograph of the different Butler matrices. ... 132 Figure 4-16 Results of BM I: (a) Simulated transmission (from port 1 and 2 to output ports);

measured transmission (b) from ports 1 and 2 and (c) from ports 3 and 4 to output ports; (d) simulated and measured reflection and transmission between ports 1 and 2. Results of BMII:

(e) Simulated transmission (from port 1 and 2 to output ports). Measured transmission (f) from ports 1 and 2 and (g) from ports 3 and 4 to output ports. (h) simulated and measured reflection and transmission between ports 1 and 2. ... 134 Figure 4-17 Results of BM III: (a) Simulated transmission (from port 1 and 2 to output ports);

measured transmission (b) from ports 1 and 2 and (c) from ports 3 and 4 to output ports; (d) simulated and measured reflection and transmission between ports 1 and 2. Results of BMIV:

(e) Simulated transmission (from port 1 and 2 to output ports); (f) simulated reflection and

transmission between ports 1 and 2. ... 135

Figure 4-18 Normalized radiation pattern ,in the elevation plane, of a 4-patch-antennas array

associated to the different fabricated Butler matrix. ... 136

Figure 4-19 Simulated and measured S-parameters (dB) of one vertical antennas array (A1) making up the total cylindrical antenna array (inside graph). The frequency of operation of the antennas is 2.42 GHz. ... 137 Figure 4-20 Simulated 3D gain patterns of the 4 ports composing an antenna array associated to an ideal 4x4 Butler matrix: (a) port 2, (b) port 4, (c) port 1 and (d) port 3. ... 138 Figure 4-21 3D view of the simulated (a) single flat antenna array, (b) single bent antenna array and (c) 3D bent structure composed of multiple antenna arrays. ... 138 Figure 4-22 Measured and simulated gain pattern (dBi) (a) of one multidirectional antenna array (A1 associated to the BM) in the E-plane, as a function of the incident angle G (°) and (b) of beam B4 in the H-plane ( G . GH/1 of the whole cylindrical antenna array. f = 2.42 GHz.

... 140 Figure 4-23 Dipole antenna (a) front view, (b) bottom view. ... 141 Figure 4-24 Simulated and measured (a) reflection coefficient and radiation pattern in the (b) E-plane and (c) H-plane of the dipole antenna. ... 141 Figure 4-25 Experimental (a) rectification efficiency (%) and (b) DC power (

W ) of the rectifier (inside graph) associated to the proposed antenna array + beam-forming (R3 and R4) and associated to a dipole antenna (multiplied by 3 for the DC power) and a patch antenna (multiplied by 4 for the DC power) as a function of the theoritical available ambient power density (µW.cm

) at the frequency of 2.42 GHz. ... 142 Figure 4-26 Proposed rectenna with one Beam-Forming Network (BFN). ... 143 Figure 4-27 Measurement setup in the anechoic chamber. ... 144 Figure 4-28 Experimental (a) rectification efficiency pattern (%) and (b) DC power pattern (dBm) in the E-plane, as a function of the incident angle G( (°), of rectifiers R1, R2, R3 and R4 of one multidirectional rectenna and of a dipole rectenna. The power density and frequency of the incident radiation are 0.45

W.cm-2 and 2.42 GHz, respectively. ... 145 Figure 4-29 DC Power management of the proposed system for zero losses. ... 146 Figure 4-30 Measurement setup in the corridor. ... 147 Figure 4-31 Experimental DC voltage (dBV) at the output of the rectifier (open-circuit and R

=5.1k

) associated to the proposed antenna array + beam-forning (R4) and associated to a dipole antenna, (a) as a function of the power density (µW.cm

) and (b) as a function of the distance (m) under 2 FCC-regulated emitting powers of 100 mW and 2W. f=2.42GHz. ... 147 Figure 4-32 Experimental DC voltage (dBV) at the output of the rectifier (open-circuit and R

=5.1k

) associated to the proposed antenna array + beam-forning (R4) and associated to a dipole antenna (+ multiplied by 3), (a) as a function of the power density (µW.cm

) and (b) as a function of the distance (m) under 2 FCC-regulated emitting powers of 100 mW and 2W.

f=2.42GHz. ... 148

Figure 4-33 Output DC power ( " W) in ambient scenarios, normalized at 2.45 GHz, of the

proposed system compared to the state-of-the-art rectennas and its counterpart DC combining

rectennas (“3-dipole” and “4-patch” rectennas) at the minimum power density ( " W.cm

)

resulting in an output DC voltage of 165 mV across the optimal load. ... 150

Figure 5-1 (a) Series and (b) parallel connection of N rectifiers. ... 153 Figure 5-2 Rectenna unit-cell (RU): (a) rectifying network (bottom of the RU), (b) antenna (top of the RU). (c) Different layers of the RU. ... 155 Figure 5-3 Transmission lines model of a half hybrid coupler. ... 156 Figure 5-4 Block diagrams of different possible configurations of the adaptive beam-forming network with a 4-edges rectenna unit-cell. The numbers and letters represent the input and output ports. ... 157 Figure 5-5 Aggregate array factors of (a) 2-RUs in the plane I( = 0 J and of (b) 4-RUs in the plane I( = 45 J . ... 160 Figure 5-6 Relation between the losses introduced by the Beam-Forming Network and the scan efficiency for M*N = 2 and 4 ( D<$9KF . 9 ). ... 161 Figure 5-7 Photograph of the rectifier. ... 162 Figure 5-8 Measured reflection coefficient (dB) of the rectifier for various input power. ... 163 Figure 5-9 (a) Simulated and measured rectification efficiency of the rectifier at 2.4 and 2.42 GHz. (b) Comparison of the experimental rectification efficiency with simulation of the Spice model with breakdown voltages of 2, 4 and 6 V. ... 163 Figure 5-10 Simulated magnitude of the hybrid coupler: (a) reflection isolation and transmission coefficients and (b) zoomed transmission coefficients. ... 164 Figure 5-11 (a) 3D view of the Momentum simulation and (b) Momentum symbol of one RNU.

... 165

Figure 5-12 Measured reflection coefficient (dB) of the rectifying unit cell (RU1). ... 165

Figure 5-13 (a) Measured rectification efficiency the rectifying unit cell and the rectifier alone

as a function of the frequency for various input powers. (b) Insertion losses of the rectifying

network. ... 166

Figure 5-14 3D view of the Momentum simulation of (a) 2 and (4) RNUs. ... 166

Figure 5-15 (a) 2 RNUs simulation: transmission from input ports 1 and 3 to output ports and

isolation/reflection coefficients.(b) 4 RNUs simulation: transmission from input ports 1 and 3

to output ports and isolation/reflection coefficients. ... 167

Figure 5-16 Assembly of (a) 2 RUs (RNU2 and RNU3) and (b) 4 RNUs (RNU4, RNU5, RNU6

and RNU7). ... 168

Figure 5-17 Reflection and isolation coefficient of (a) two RNUs (RNU2 and RNU3) associated

and (b) 4 RNUs (RNU4, RNU5, RNU6 and RNU7) associated. ... 169

Figure 5-18 Patch antenna. ... 169

Figure 5-19 Experimental (a) reflection coefficient (dB) and (b) radiation pattern in the E- and

H-planes at 2.4 GHz. ... 170

Figure 5-20 Photograph of (a) the measurement setup of the equal distribution of power into

the rectifiers and of (b) the DC connection of the rectifiers’ outputs. ... 171

Figure 5-21 (a) DC output power ( " W) versus the input power (dBm) of 4 rectifiers (R1, R2,

R3 and R4), of 2 series-connected rectifiers (R3+R4) and of 4 series-connected rectifiers

(R1+R2+R3+R4). (b) DC combination efficiency (%) for the DC series connection of 2 and 4

rectifiers. ... 171

Figure 5-22 Measurement setup for the (a) DC combination of 2 RNUs and for (b) the RF

combination of 2 RNUs. ... 172

Figure 5-23 (a) Measured rectification efficiency (%) and (b) DC voltage at the output of RNU2,

RNU3, RNU2 and RNU3 in series and R3 and R4 in series at 2.42 GHz. ... 173

Figure 5-24 Power and voltage gain factors at 2.42 GHz of 2 RNUs. ... 174

Figure 5-25 Measurement setup for the (a) DC combination of 4 RNUs and for (b) the RF

combination of 4 RNUs. ... 174

Figure 5-26 (a) Measured rectification efficiency (%) and (b) DC voltage at the output of RNU4,

RNU5, RNU6, RNU7 and RNU4, RNU5, RNU6 and RNU7 in series and R1, R2, R3 and R4 in

series at 2.4 GHz. ... 175

Figure 5-27 Power and voltage gain factors at 2.4 GHz of 4 RNUs. ... 176

Figure 5-28 Measurement setup in the anechoic chamber of Poly-Grames, Ecole Polytechnique

de Montréal. ... 176

Figure 5-29 Power and voltage gain factors at 2.42 GHz of 2 RUs. ... 177

Figure 5-30 Measured (a) rectification efficiency (%) as a function of the load (k # ) for various

input powers for (a) 2 RUs and (b) 4 RUs. ... 178

Figure 5-31 Measured (a) DC output voltage (V) and (b) DC power (µW) as a function of the

available power density (µw.cm

) of rectennas made up of 1, 2 and 4 RUs DC-connected in

series with a resistive load of 5.1, 10 and 20 k

respectively. ... 178

Figure 5-32 Laboratory measurement setup of orientated communication and localization of

RF-connected RUs. ... 179

Figure 5-33 (a) Measured received and demodulated signal at the output of the two RUs RF

connected (RU2 on top and RU3 at the bottom) for different distances d of the tag from the

center of the cage (in front of the reader) to its edges. (b) Normalized voltage at the output of

RU2 and RU3 (top graph) and voltage difference between the output of RU2 and RU3 (bottom

graph) as a function of the distance d of the tag from the center of the cage. ... 180

Figure 6-1 Prospect of the output DC power ( " W) in ambient scenarios (normalized at 2.45

GHz), that can be envisioned by combining the proposed system of Chapter 4 and the

reconfigurable system presented in Chapter 5. From Figure 4-33. ... 185

List of Tables

Table 1-1 Ambient Power densities Measurements ... 26

Table 1-2 Frequency Spectrum and Allowed Power Levels for UHF RFID and WLAN ... 27

Table 2-1. Spice Parameters of Schottky Diodes ... 50

Table 2-2 Parasitic Elements of Diode’s Packages ... 54

Table 2-3. Parasitic Parameters of the SOT-23 Package of Schottky diodes SMS7630 and SMS7621 ... 57

Table 2-4 DC Power Collected by Rectennas in Different Scenarios ... 73

Table 2-5 Polarization Mismatch. ... 76

Table 2-6 Polarization Mismatch Over Time in a Rayleigh Propagation Environment. ... 77

Table 3-1 Antenna’s dimensions. ... 88

Table 3-2 Dielectric Properties of the R04003 and Paper Substrates @ 2.45 GHz ... 91

Table 3-3 Antenna’s dimensions. ... 92

Table 3-4 Influence of the air gap on the dielectric properties of the antenna’s substrate. ... 92

Table 3-5 Solid angle, aperture efficiency and average polarization mismatch factor of the antennas of section 3.1 ... 98

Table 3-6 Metrics of the different possible rectennas at the sensitivity of the rectifier (for a power of -17.6 dB at the input of the rectifier operating with an efficiency of 30.6 %) ... 103

Table 4-1 Maximum Insertion Losses of a 4x4 Butler Matrix for Energy Harvesting ... 123

Table 4-2 Maximum Insertion Losses of a NxN Butler Matrix for Energy Harvesting ... 125

Table 4-3 Phase Differences at the Output Ports of BMI and BMII ... 133

Table 4-4 Phase Differences at the Output Ports of BMIII and BMIV ... 135

Table 4-5 Comparison of 4x4 Butler Matrices ... 135

Table 4-6 Maximum Gain in the Elevation Plane of the Patch Antennas of the Array A1 .. 139

Table 4-7 Comparison of the Proposed Rectenna with the State-of-the-Art for Ambient RF Energy Harvesting ... 149

Table 5-1 Efficiency and Benefits of Series and Parallel DC Combination of Rectifiers. ... 154

Table 5-2 Phase Differences at the Output Ports of 2 RNUs. ... 167

Table 5-3 Phase Differences at the Output Ports of 4 RNUs. ... 167

Table A.1-0-1 Relative permittivity and loss tangent of low-loss dielectric ROGERS laminates ... 188

Table A.4-0-2 Dielectric Properties of the RO4003 and Paper Substrates ... 202

List of Acronyms and Abbreviations

RF Radio Frequency

DC Direct Current

EM Electromagnetic

IoT Internet-of-Things

ITU International Telecommunication Union

5G Fifth Generation (of Cellular Network)

EH Energy Harvesting

WPT Wireless Power Transfer

LF Low Frequency

HF High Frequency

UHF Ultra-High Frequency

WEH Wireless Energy Harvesting

WPCN Wireless Powered Communications Network

SWIPT Simultaneous Wireless Information and

Power Transfer

WSN Wireless Sensor Network

TX Transmit

RX Receive

EV Electrical Vehicles

GSM Global System for Mobile Communications

WiFi Wireless Fidelity

DTV Digital Television

UMTS Universal Mobile Telecommunications

System

EIRP Effective Isotropically Radiated Power

ERP Effective Radiated Power

RFID Radio Frequency Identification

ETSI European Telecommunications Standards

Institute

TPC Transmit Power Control

WLAN Wireless Local Area Network

MPPT Maximum Power Point Tracking

LTE Long-Term Evolution

UWB Ultra-Wide Band

FSS Frequency Selective Surface

RFOM Rectenna Figure-of-Merit

RTI Rectenna Topology Indicator

BFN Beam-Forming Network

PCE Power Conversion Efficiency

ADS Advanced Design System

LSSP Large Signal S-Parameters

VNA Vector Network Analyzer

TRL Trough-Reflect-Line

LH Horizontally

RH Vertically

V Vertically

H Horizontally

DP Dual Polarized

CP Circular Polarized

RHC Right-Handed Circular

LHC Left-Handed Circular

RHE Right-Handed Elliptical

LHE Right-Handed Elliptical

FoM Figure-of-Merit

3D 3-Dimensional

PEC Perfect Electric Conductor

CST Computer Simulation Technology

E Electric Field

H Magnetic Field

HPBW Half-Power Beam Width

TI Texas Instruments

SMA SubMiniature version A

BM Butler Matrix

BALUN Balanced to Unbalanced

RU Rectenna Unit

RNU Rectifying Network Unit

PGF Power Gain Factor

VGF Voltage Gain Factor

!

!

Chapter 1 General Introduction

!

1 ! General Introduction

This chapter first introduces the trends and needs for wireless links in the power supply of Radio Frequency (RF) communicating, sensing and actuating systems. The need for battery- less devices as well as passive operations has led to the concept of energy harvesting, and particularly wireless (or electromagnetic) energy harvesting and passive wireless communications techniques. The implementation of wireless power transfer is described here in different possible scenarios. The second part of this chapter focuses on the key element of a far- field power receiver, namely the rectenna or rectifying antenna, which is expected to be embedded in a RF receiver without any disturbance on it and to have low environmental impact.

The rectenna collects intentionally transmitted or ambient RF power and converts it into useful Direct Current (DC) power, compatible with the current electronic design and battery- operation. In the third part, the different challenges regarding the rectenna design for low-power operation are discussed and the state-of-the-art is described. Metrics, reported in the literature, are also given for the suggestion of the most appropriate topology of rectifiers/rectennas.

Finally, the contribution of this work is depicted: antenna array techniques and innovative

rectenna prototypes are proposed for the development of ambient energy harvesting or sensing

applications.

Chapter 1 General Introduction

1.1 ! General Context: The Need for Battery-less Sensors and Passive Sensing Applications

1.1.1 ! The “Wireless Trends”

Wireless communications using Electromagnetic (EM) waves have been exponentially developed throughout the 20th century, and are now a common way, all around the world, of sharing and transmitting information. Nowadays, wireless communications are globally experienced everyday by billions of individuals on earth with smartphones, laptops or all other connected objects. These communication trends are not going to stop anytime soon: the number of cellphone subscribers keeps growing

in November 2018 [1]. During the last decades, the number of connected objects has also exploded.

Up to 29 billions of connected objects are forecast in 2022 [1], 18 billions of which will be equipped with sensors and network links in order to capture and share information thanks to their internet connection for Internet-of-Things (IoT) applications.

The concept of the Internet-of-Things [2] was first introduced in 1982 but has matured over the last decades and was finally defined in 2012 by the ITU (International Telecommunication Union) in Recommendation ITU-T Y.2060 [3] as “a global infrastructure for the information society, enabling advanced services by interconnecting (physical and virtual) things based on existing and evolving interoperable information and communication technologies”. In other words, it is the connection to the internet of small scattered devices, representing the physical world, allowing for the gathering of information and the communication of this information.

Figure 1-1 2018 Gartner hype curve for emerging technologies [4].

Chapter 1 General Introduction

!

The applications of the IoT concern a lot of areas and aim at changing the humans’ ways life for a better comfort (health, agriculture, …) but also for a more sparing way of consuming energy. This is partially achieved by transforming our environment into smart environments (smart cities, smart buildings, smart utilities, smart cars, etc.). The IoT market represents hundreds of billions US $ [5], and it will continue growing in the future years. Figure 1-1 shows the Gartner hype curve of 2018 [4]: while the connected homes and the IoT platform are already in the peak of inflated expectations, some other IoT applications such as smart workspace or smart robots are innovation triggers, as well as technologies serving the IoT such as the blockchain for data security and the AI.

As the number of connected items is considerably increasing, especially with the promises of the fifth generation (5G) of cellular network technology, the use of a multitude of sensors or tags geographically distributed is required to sense and share information. Those devices are, most of the time, embedded in the items and their power supply cannot be realized with wired connections. The deployment of a large number of sensors/tags is indeed dependent on their autonomous operation or their wireless power supply, leading to the requirement for battery- less scenarios or prolonged battery-life [6]–[9]. The sensors must be able to collect energy from their environment or from an intentional energy or power source thanks to integrated wireless energy harvesters, expected to be very little cumbersome and to have low economic and environmental impacts. This is possible with the use of flexible, low-cost, recyclable and circular economy compatible materials. The main reason for the necessity of energy harvesters is the cost of maintenance and replacement of the batteries and the difficulty of their replacement when the sensors are placed in hardly reachable locations. There is also a will to limit the number of deployed batteries to reduce the ecological footprint. In consequence, a lot of effort has been furnished for those small electronics apparatuses to consume very low power, allowing their power supply from their environment, and wireless solutions have been developed to obtain battery-less operations. Nowadays, besides the data, the power needs to be wirelessly transferred and energy needs to be harvested.

1.1.2 ! Wireless Power Transfer (WPT) and Wireless Energy Harvesting (WEH) Definitions

The power supply of an electronic device basically relies on the ability of this device to collect

energy in its environment without wired connection, which is conventionally referred to as

Energy Harvesting (EH) [10], [11]. In a general way, the concept of energy harvesting represents

the scavenging of the energy present in the device’s environment such as solar, chemical,

thermal, mechanical or electromagnetic energy, whether the source is intentionally generated

by humans or not. This energy can be directly used to power the item’s operation or can be

stored for battery support. The power supply of a device with Electromagnetic (EM) energy is

Chapter 1 General Introduction

!

generally referred to as Wireless Power Transfer (WPT) [12]. The possibility to transfer energy between two points without physical contact, as suggested by the Maxwell’s equations since the 19

century, has been revealed by the experiences of Heinrich Hertz from 1886 to 1888. Eleven years later, Nicolas Tesla ran experiments where he attempted to transmit power of up to 300 kW at the frequency of 150 kHz, to demonstrate the possibility of wireless power transfer [13].

One of his dream was the distribution of wireless electric power and data all over the world.

Depending on the frequency of the EM wave, two types of WPT can be categorized as follows:

•!

Non-radiative transfer (near-field region) that uses inductive/capacitive coupling or coupled magnetic resonance to transmit data at Low Frequency (LF) and High Frequency (HF) frequency bands corresponding to frequencies from 3 kHz up to 30 MHz.

•!

Radiative transfer (far-field region) at Ultra High Frequency (UHF) and microwave frequencies from 300 MHz and up to 100 GHz.

Note that the far-field region at frequency , . 8LM corresponds to distances between the emitter and the receiver greater than the Fraunhofer distance =

defined as

=

N O<

M (1.1)

where D is the maximum length of the biggest antenna among the emitter and receiver.

In the near-field region, the non-radiative WPT involves inductive/capacitive coupling and resonant inductive (or magnetic) coupling to transmit wireless power in the range of a few centimeters with inductive/capacitive coupling and up to a few meters with resonant inductive coupling. Inductive coupling, for example, is used for the wireless charge of batteries of toothbrush, cellphones, or medical implants [14] etc. It consists of two coils close to each other that exchange power thanks to the principle of induction. The flow of an AC current on a coil creates a magnetic field varying in time inside the coil. As suggested by the Faraday’s law given by (1.2), this gives rise an electromotive force (emf).

P Q R .(? ST

SU (V(W RX YZ

#

. ? [

[U W TX Y\

. +5, (1.2)

When two coils are close to each other, the emf induced by the current flow in one of the coil results in a current flow in the other coil. The power transfer is maximized by matching the impedance of the emitter to that of the receiver.

Resonant inductive coupling uses intermediate resonant circuits to transfer power more

efficiently, as shown in Figure 1-2. This technique is the most utilized technique for various

Chapter 1 General Introduction applications including the charge of electric vehicles’ batteries [15]. Resonant inductive coupling consists of adding elements to coils so that a high-quality factor resonance is created at a given frequency. Thus, when two resonant circuits closed to each other resonate at the same frequency, the power is transferred with high efficiency. The efficiency of this wireless power transfer depends on the quality factors and coupling factors of the different circuits.

Figure 1-2 Non-radiative wireless power transfer in the near-field region with resonant inductive coupling.

Figure 1-3 Radiative wireless power transfer in the far-field region.

In the case of a radiative power transfer in the far-field region, depicted in Figure 1-3, the electric and magnetic fields propagate as an electromagnetic wave whose behavior is described by (1.3) and (1.4), derived from Ampere’s and Faraday’s laws.

] Q R . ?"

S^

SU (1.3)

] Q ^ . _

SR

SU (1.4)

The power density traveled by this electromagnetic wave is given by the Poynting vector:

Chapter 1 General Introduction

` . 9

O R a ^ (1.5)

The average power density over time reaching the receiving antenna, S, can be expressed as:

; .( b`b . 9

8X "

c

(1.6)

The collection of EM power by a device is called electromagnetic or wireless energy harvesting (WEH) and usually concerns radiative energy transfers in the Radio Frequency (RF) or microwave domains. RF WEH is a promising solution to develop self-powered electronics.

Indeed, the robustness of RF energy and its omnipresence, mostly due to human activities, embracing TV/radio broadcasting, wireless communications and sensing, makes it a good candidate for supplying electrical power to low-power and low-duty cycle electronic devices. In fact, when power-constrained networks with slow activity rates, are at stake, the less costly and simpler solution is to recycle RF power that is present in the environment over a certain time.

However, sometimes the exploitation of installations for RF communications is more efficient since a larger amount of power can be transmitted. The presence of base station facilitates the wireless power supply of electronic devices. In this case, the power is intentionally sent to the item and harvested by the item, with WPT.

Thus, we can distinct two scenarios of RF WEH, in which the energy is harvested from 2 possible sources:

•!

Ambient RF sources

•!

Base Stations or “RF showers” (radiative WPT) [16]

Figure 1-4 shows a block diagram of the definitions given in this section.

Figure 1-4 Block diagram of the definitions of Wireless Power Transfer (WPT) and Wireless

Energy Harvesting (WEH).

Chapter 1 General Introduction

1.1.3 ! Towards Battery-less Sensors and Passive Sensing Applications

WEH is implemented in power-constrained networks, such as wireless sensors. Figure 1-5 illustrates a smart city in which examples of applications using IoT wireless sensors are depicted.

As mentioned in the previous subsection, EM energy can be harvested from an intentional source with radiative WPT or from ambient RF sources. We can distinct several scenarios of how WEH is used and which EM source is exploited.

Figure 1-5 Smart city equipped with IOT wireless sensors.

When wireless sensors communicate with a base station, the sensors can collect RF energy from the base station, in addition to data, through an energy harvester embedded in the receiver [17]. The transfer of power by a base station associated to its data transmission is referred to as Wireless Powered Communications Network (WPCN) while the simultaneous transmission of data and power is referred to as Simultaneously Wireless Information and Power Transfer (SWIPT) [18]–[20]. Information and energy can be collected from a base station in the downlink by using two separate receivers with various different techniques [18], [19], such as: time switching, power splitting, antenna switching or spatial splitting. Figure 1-6 illustrates both SWIPT and WPCN scenarios.

Figure 1-6 Wirelessly powered sensors communicating with a base station in (a) a SWIPT and

(b) a WPCN scenarios.

Chapter 1 General Introduction Another scenario is the wireless power supply of actuating systems, which do not communicate.

The wireless power transfer and ambient energy harvesting enable the wireless charging of actuating systems such as medical implants (pacemakers), IoT devices or other kinds of consumer devices.

Figure 1-7 Wireless power supply of (a) low-duty cycle and low-power sensors and (b) backscattering sensors.

WEH is also used for the power supply or the charging of low-duty cycle and low-power Wireless Sensor Nodes (WSNs).

In these two scenarios, only power is wirelessly transmitted by a base station to the device or ambient energy in the sensor’s environment is recycled, as shown in Figure 1-7 (a). Note that, the non-radiative WPT techniques are used for low power networks only, while the transfer of higher quantity of energy is performed with non-radiative techniques (like for Electrical Vehicles (EVs) for example).

There are three parts in the wireless sensors’ power usage: sensing, data processing, and communication. Because the communication is the most power consuming process, wireless solutions such as the backscattering communication technique [21]–[25] can be exploited to obtain passive sensing operations such as wireless sensing [26], [27], detection, identification and localization [16]. This solution consists of sending a carrier signal with a base sation (reader) to a sensor that modulates the reflection with information. The scatter radio technique, also referred to as backscattering, enables the back-transmission of the information with only the modulation of the reflection to be powered (no power-hungry signal generation involved). There are three types of backscattering techniques:

•!

monostatic backscattering: a reader sends and receives the carrier and the tag modulates

•!

bistatic: an emitter sends the carrier; a receiver exploits the signal modulated by the tag

•!

ambient backscattering [21], [22]: ambient signals act as the carrier emitter; a receiver, at the same frequency as the ambient signals, recover the signal modulated by the tag.

In this particular case, the range is very limited due to high TX-RX leakage.

Chapter 1 General Introduction Furthermore, RF power can be harvested by the sensor to modulate the signal, as shown in Figure 1-7 (b); and therefore a complete passive sensor can be obtained [16], [25]. Another technique involving any chip for the modulation of information, can also be implemented by using a tags/sensor directly sensitive to the information to sense and able to reflect back that

In conclusion, chipless sensors need to be developed or WEH systems need to be embedded in RF receivers in order to form battery-less (possibly multi-function) receivers and allows for wireless passive sensing applications embracing sensing, detection, identification, localization, etc. Moreover, practical implementation of WEH systems on RF receivers (in large numbers) of various geometric forms requires the necessity to develop low-cost WEH systems with a high degree of recyclability among other characteristics such as flexibility, lightweight, etc.

1.2 ! Wireless Energy Harvesting (WEH)

As mentioned in section 1.1, the low-power operation of electronic devices enables the collection of wireless energy either from a remote RF power source or from their environment as a mean of supply in order to obtain sustainable, passive/autonomous and maintenance-free systems.

Figure 1-8 illustrates the two scenarios of WEH: ambient EH and far-field WPT. EM energy is harvested with a rectenna, which allows for the capture of EM power and its conversion into DC power. This section gives some information on the EM energy that can actually be harvested and on the tools implemented to harvest this EM energy.

Figure 1-8 The two scenarios of RF energy harvesting: (a) the energy is harvested from a RF source that intentionally transmits power to the harvester (b) harvesting ambient RF power

present in the environment.

1.2.1 ! Ambient Power Densities

The electromagnetic energy can be found in the environment within a large spectrum from

the sun light to the radio waves. Although the omnipresence and the momentary superposition

of some of the RF radiations of different frequencies tend to enhance the available ambient

Chapter 1 General Introduction

!

power densities, the amount of energy will always be limited for safety reasons. Table 1-1 reports some ambient RF power densities measurements [30]–[34], mostly performed in urban areas.

These results have to be examined carefully since the quantities of energy vary a lot with time and location as the number of users and their activities differ from one place/period of time to another. Moreover, the rules about the electromagnetic expositions strongly vary from one country to another as well as the frequency bands allocated to the different applications.

Nevertheless, in general, between 10 nW.cm

and 100 nW.cm

can be expected at some frequencies of RF communications, most often GSM900 (downlink in Europe: 935–960 MHz), GSM1800 (downlink in Europe: 805.2–1879.8 MHz) and WiFi (2.4-2.5 GHz) (in indoor environments particularly) bands.

Table 1-1 Ambient Power densities Measurements

Power Density (nW.cm^-2)

Frequency Band DTV

400-800 MHz

GSM900 900 MHz

GSM1800 1800 MHz

UMTS 3G 2100 MHz

Wi-Fi 2400 MHz Average [4] London, UK

Average [7] Covilha, Portugal Median [8] France

Average [5] Austria, Germany and Hungary, 25 to 100 m from base stations

0.89 0.34 68

-

36 2.79

44 10-100

84 1.2 44 -

12 1.25

51 -

0.18 - - -

1.2.2 ! Power Regulations

In each country, regulations exist to limit the transmitted wireless power, mostly for safety reasons. The maximum allowed transmission power is given in terms of the maximum EIRP (Effective Isotropic Radiated Power) of the radio power source, defined as [35]:

cd=) . )

X e

(1.7)

where )

is the emitted power and e

is the gain of the radio emitter. In some cases, instead of EIRP, ERP is given. ERP (Effective Radiated Power) is the total power that would be emitted with a standard half-wave dipole antenna of gain 2.15 dBi (1.64). Thus EIRP can also be expressed as:

cd=) . 9Xf/ g c=) (1.8)

The available power density at the receiver level can be then determined from EIRP of the

source with:

Chapter 1 General Introduction

!

; . cd=)

/0=

(1.9)

As an example of power regulations, Table 1-2 gives the maximum allowed power levels in some regions for the UHF frequency bands allocated for RFID (Radio-Frequency Identification) applications and WiFi transmissions.

Table 1-2 Frequency Spectrum and Allowed Power Levels for UHF RFID and WLAN

Europe 869.4 to 869.65 MHz 865 to 868 MHz 865.6 to 867.6 MHz

865.6 to 868 MHz

500 mW ERP 100 mW ERP

2 W ERP 500 mW ERP

America 902 to 928 MHz 4 W EIRP

Japan 952 to 954 MHz 4 W EIRP

Korea 908.5 to 914 MHz 4 W EIRP

Australia 915 to 928 MHz 1 W EIRP

Europe, Middle East, Africa, China, Indonesia,

Singapore, Thailand, Vietnam, Part of Russia

(ETSI standard)

2400 – 2483.5 MHz

5150 to 5350 MHz 5470 to 5725 MHz 5725 to 5875 MHz

100 mW

200 mW 1 W 4 W

America 2400 – 2483.5 MHz

5150 to 5250 MHz 5250 to 5350 MHz 5725 to 5850 MHz

1 W 200 mW

1 W 4 W

A RFID reader delivers a power from 100 mW to 4W EIRP depending on the region, application and frequency bands. In a big part of the world, the WiFi’s maximum power is regulated by the ETSI (European Telecommunications Standards Institute) standard, which fixes the maximum power to specific channels. For most of countries over the world, the power limitation varies from 100 mW to 4 W as well, depending on frequency band of interest. Note that the ETSI standard’s maximum power is given for emitters equipped of the TPC (Transmit Power Control) feature that reduces the power of emitter to the minimum necessary to maintain the link with a certain quality. Since the frequency bands allocated to the WiFi slightly differ from the regions concerned by the ETSI standard and the regions not concerned such as the USA, it is generally more rigorous to talk about the channels rather than the frequency bands.

Nevertheless, the purpose of Table 1-2 is just to give an insight of the power we deal with, at

the source, when harvesting RF power in the far-field region.

Chapter 1 General Introduction

1.2.3 ! Rectenna

First introduced by Brown in the 1960’s [36], the rectenna had been largely developed for point- to-point transmissions involving high power of several Watts. Figure 1-9 shows a photograph of the first rectenna built in 1963 and made up of 28 half-wave dipoles terminated by full-bridge rectifiers [36]. The rectenna was able to deliver 4W of DC power with a 50% efficiency. Since then, the operation of rectennas has been pushed to much lower power levels imposing new challenges. The frequency of operation is also being increased to the K band (18-27 GHz) [37]–

[40], the Ka band (26.5-40 GHz) [41], the E band (60-90 GHz)[41], [42] and even the sub-THz band [43].

Figure 1-9 Photograph of the first rectenna (1963) made up of half-wave dipoles each terminated with full bridge rectifiers [36].

Following the huge attention brought to ambient wireless energy harvesting in the two last decades, the rectenna for low-power levels has been largely investigated with respect to the commercial bands from 100 MHz to 5.8 GHz. A rectenna for low-power applications, as shown in Figure 1-10, is composed of an antenna impedance-matched to a rectifier, which is usually followed by a DC power management unit that regulates the DC output voltage of the rectifier.

Since this voltage is commonly low in WEH applications because of the low ambient power

densities or the need for extending the power transmission range, the DC-DC converter is

usually used to boost the output DC voltage for storage or power supply. A low-pass filter is

commonly placed between the antenna and to prevent harmonics generated by the diode from

being reemitted by the antenna above mW power operation.