Hardware Architecture of Wireless Power Transfer, RFID, and WIPT Systems
HHardware Architecture of Wireless Power Transfer, RFID, and WIPTSystems
Yu Luo ∗ , Lina Pu †∗ ECE Department, Mississippi State University, Mississippi State, MS, 39759 † Department of Computer Science, University of Alabama, Tuscaloosa, AL, 35487Email: [email protected], [email protected]. H
ISTORICAL M ILESTONES OF W IRELESS P OWER T RANSFER (WPT)The early effort on WPT can be traced back to late 1950swhen a theoretical analysis from Goubau and Schweringshowed that power could be transmitted over any distancewith near efficiency through a concentrated beam [1].Three years later, this theory was confirmed by an experi-mental demonstration indicating the born of an effective WPTsystem [2]. Around 1963, the rectenna was invented, which is amemorable event in the history of the WPT development. Therectenna can efficiently convert electromagnetic (EM) energyarrived at a WPT receiver into the direct current (DC). Theinvention of the rectenna paved the way of long-distance WPT.In the early stage of WPT development, a representativeexperiment was to power an unmanned helicopter with the mi-crowave energy reflected from an ellipsoidal reflector placed atthe ground [3] . The small helicopter was able to hover severalmeters above the reflector without extra energy supply [4].With an increased energy conversion efficiency in . – . GHz frequency bands, the flight altitude of the microwave-powered aircraft was further improved. In 1988, CanadianStationary High Altitude Relay Platform Program (SHARP)demonstrated an airplane with . m wingspan. The airplanecould fly at m altitude for . minutes through receivingenergy radiated from a parabolic dish with kW transmissionpower [5].In 1968, Peter Glaser introduced a concept of solar powersatellite (SPS) that captures solar power through the satelliterunning in the geostationary orbit, and then sends the harvestedenergy back to the earth via microwaves [6]. To examinethe feasibility of a long range WPT from a satellite to theground, a successful experiment was conducted by Raytheoncompany in 1975. Between 1978 and 1986, DOE (Departmentof Energy) and NASA (National Aeronautics And SpaceAdministration) jointly investigated the satellite power systemconcept development and evaluation program [7]. Nowadays,SPS is still considered as one of potential candidates to replacethe fossil fuel and nuclear energy for a green, safe, andsustainable power supply [8].II. H ISTORICAL M ILESTONES OF W IRELESS I NFORMATION T RANSFER (WIT)Since Maxwell’s equations was published and later verifiedby Heinrich Hertz in 1888, the wireless communications haswell prepared to walk into the real world. In 1902 and 1914, the frequency modulation (FM) and the amplitude modulation(AM) was proposed respectively for wireless communications.Until today, those two modulation schemes are still widelyused for the radio broadcasting.1948 is a year worth remembering for us. In this year,Claude E. Shannon laid out the foundation of informationtheory in his paper entitled “A mathematical theory of commu-nication” [9]. In this paper, Shannon for the first time definedthe channel capacity and described it with a mathematicalmodel. Thereafter, researchers have a mathematical model toaccurately calculate the fundamental limits on the wirelesscommunication channel.In 1979, the first generation (1G) of wireless cellular tech-nology was launched in Japan. It applied the analog telecom-munication technology, and then replaced by the secondgeneration (2G) of digital telecommunication technology. Inlate 1991, GSM (global system for mobile communications),which was the world first TDMA (time-division multipleaccess) standard, was developed for 2G networks [10].Today, we are experiencing the high speed fourth-generationlong term evolution (4G LTE) broadband cellular network,and expecting for more surprise, like the virtual reality (VR)and the edge computing, that will be brought by the fifth-generation (5G) mobile network in the near future. Comparedwith the 2G GPRS (general packet radio service), whichprovides up to kbps of download and kbps of uploaddata rates, the 4G network supports up to Mbps and Gbpsdata rates for applications with high mobility and low mobility,respectively [11]. These two rates will turn out to be at leastten times higher in the incoming 5G communications [12].III. H
ISTORY OF W IRELESS I NFORMATION AND P OWER T RANSFER (WIPT)The original developments of WPT systems and WIT sys-tems are independent. The former had a huge size in order toachieve an efficient energy transfer at a high-power level [13],while the latter aimed to reduce the device size for high-speed communications with a low power consumption. Until1948, Harry Stockman introduced a new concept, called thecommunication by means of reflected power [14]. His workis the prototype of the radio frequency identification (RFID),which is the first attempt to combine WIT with WPT.Three decades later, a short-range RFID system was im-plemented based on the modulated backscatter [15]. In thissystem, the RF reader sent a W of continuous wave (CW) a r X i v : . [ c s . A R ] F e b t GHz to an RF tag, which then changed the load onits rectifier to modulate the amplitude of backscatter wavesfor the data transmission. Since a portion of CW energyfrom the RF reader was rectified to power the RF tag forwireless communications, the passive RFID, can be consideredas the first WIPT system in the history. Nowadays, RFIDand its derivatives, near-field communication (NFC) [16], havebeen widely used in different places for item classification,electronic payment, and tool management [17], [18].The invention of integrated circuit (IC) further promoted theintegration of WPT and WIT [19]. With the rapid developmentof the IC technique, the computational speed and the powerconsumption of semiconductor chips had a drastic improve-ment [20]. Taking Intel’s microprocessors as an example, thenormalized thermal design power (TDP) of Pentium MMX233 released in 1997 was mW/MHz/core [21]. In 2017, thisvalue was reduced to mW/MHz/core in Core i7-8700K, only . of Pentium MMX 233 [22], as shown in Fig. 1. Pentium II 450 (250 nm)Pentium III 1133 (180 nm)Pentium III 1400 (130 nm)Pentium D 960 (65 nm)Core 2 E6850 (65 nm)Core 2 Q9650 (45 nm)i7-2700K (32 nm)i7-8700K (14 nm)Pentium MMX 233 (350 nm) N o r m a l i z e d T D P o f C P U s ( W a t t / M H z / c o r e ) m W / M H z / c o r e m W / M H z / c o r e % Figure 1. With the progress of semiconductor technology, the normalizedTDP of Intel’s processors decreasesk significantly.
The low TDP of modern microprocessors makes themrealistic to perform some complicated tasks, e.g., fast Fouriertransform (FFT), coding, and decoding with an extremelylow energy consumption. In 1990s, the concept of smart dustwas proposed. As demonstrated in Fig. 2, the smart dust is aminiature wireless mote. It is constructed as an aggregationof multiple tiny microelectromechanical systems (MEMS)for environment sensing and wireless communications [23].Different from the passive RFID that obtains energy from adedicated RF reader, the smart dust can harvest energy fromsurrounding environment, such as solar, vibrations, or evenambient RF waves [24], for a perpetual operation. Owing tothe features of miniaturization and sustainability, the smartdust is gradually penetrating into our daily life [25], [26].In recent years, the idea of WIPT is attracting people’sattention [28]. In WIPT, an RF facility can transmit energy andinformation to an associated wireless devices at the same time.The scenarios with both a co-located and a separate energy andinformation receivers were explored in [29] for WIPT runningin a multiple-input and multiple-output (MIMO) network. To-day, researchers are putting efforts on integrating WIPT system We calculate the Normalized TDP = TDP of processorClock rate × Number of cores . Figure 2. A smart dust, Michigan Micro Mote (M ) developed by Universityof Michigan, sitting on a penny [27]. into the 5G massive MIMO communications. They aim totransmit the energy flow and the data flow to wireless devicesflexibly through narrow beams for efficient and sustainablewireless communications [30], [31].IV. I NSIGHT INTO
WPTThis section focuses on the WPT technology, which is thefoundation of WIPT. We first introduce the features of EMwaves, and then discuss several common WPT methods thatcan combine with the WIT technology for WIPT.
A. Features of EM Waves
The EM wave is the carrier of both energy and informationin WIPT systems. Its behavior depends upon the propagationdistance, the wavelength, and the size of the transmittingantenna. In general, an alternating EM field can be dividedinto following three regions [32], [33]:a)
Reactive near field (induction region) . The boundary ofthis region is
R < . (cid:112) D /λ , where R , D , and λ are the propagation distance, the dimension of the trans-mitting antenna, and the EM wavelength, respectively.The induction region has two critical features. First, theelectric and magnetic fields are separate. Second, if thereis no receiving device, the EM energy flows around thetransmitting antenna without any loss [34].b) Radiating near field (Fresnel region) . It is a region bountiedby . (cid:112) D /λ < R < D /λ . The induction andradiating fields coexist in the Fresnel region. Differentfrom induction field, the energy in the radiating fieldcannot be retrieved once it leaves the transmitting antenna.Moreover, the radiation pattern of an EM field in this regionvaries with the propagation distance, hence the relationshipbetween the electric field and the magnetic field is complexin the Fresnel region.c) Far field (Fraunhofer region) . When the propagation dis-tance of an EM wave is longer than D /λ , it enters thefar-field region. In this region, the radiating field is thedominator, which determines the antenna’s transmissionpattern. The electric field and the magnetic field in the farfield cannot be separated anymore but propagate togetheras a wave. In the Fraunhofer region, the electric field, themagnetic field, and the propagation direction of EM wavesre always orthogonal to each other. The radiation patternof EM waves in the far field does not change with thedistance.By utilizing the properties of EM waves in above threeregions, people developed four representative WPT methods:the near-field inductive coupling WPT, the near-field capacitivecoupling WPT, the middle-range resonant inductive couplingWPT, and the far-field radiative WPT, as depicted in Fig. 3. Infollowing sections, we give a briefly introduction on each ofthe four methods. I t I t Coil (a) I t I t (b) t VE t Antenna (d) I t I t (c)Magnetic field Electric fieldLC resonant Capacitor
Figure 3. Four different WPT methods, where I , V , t , and E represent thecurrent, the voltage, the time, and the electric field, respectively. (a) Near-field inductive coupling WPT. (b) Near-field capacitive coupling WPT. (c)Middle-range resonant inductive coupling WPT. (d) Far-field radiative WPT. B. Near-Field Inductive Coupling WPT
In the near-field inductive coupling WPT system, antennasat the sender and the receiver sides are two subtending coilsto transfer energy wirelessly through the magnetic field, asdemonstrated in Fig. 3(a). To work properly, the transmittingcoil needs to connect with an alternating current (AC) powersupply to generate an oscillating magnetic field. Accordingto the Faraday’s law, AC is generated at the receiver sideonce the oscillating magnetic field passes through the receivingcoil [35].The inductive coupling WPT can only work efficientlyin the reactive near field region. Therefore, the transmit-receive coils must be close to each other so that most of theinduction lines produced by the transmitter can extend to thereceiving coil. Furthermore, the dimension of the transmittingcoil needs to be much smaller than the wavelength of the EMfield so the energy cannot escape from the radiation of EMwaves. Therefore, in an inductive coupling WPT system, thefrequency of the AC power supply should be low and thewavelength of the EM field should be long [36], [37].The inductive coupling WPT technology has been widelyused in the application of wireless charging. For instance, theelectric toothbrush can directly use the / Hz AC from anoutlet to charge the battery; a smartphone like iPhone X andSamsung Galaxy S8 that supports the inductive mode of the Qi standard can be changed at several kilohertz frequency [39]. (a) (b) Figure 4. The Audi wireless charging system (recreated from [40]). (a) Movea vehicle to the charging place. (b) Raise the floor plate to change the vehiclewirelessly.
Today, the inductive coupling WPT has become an al-ternative to charge electric vehicles [41], [42]. To achievethis, automobile manufacturers integrate the receiving coilinto a vehicle’s chassis and the transmitting coil is placedat an independent floor plate, as demonstrated in Fig. 4. Asannounced in [40], through using the inductive AC couplingtechnology developed by Audi automobile manufacturer in2015, the wireless charging system can offer a power of . kW, and a higher power up to kW is possible in a futureversion. C. Near-Field Capacitive Coupling WPT
Different from the inductive coupling method, which utilizesthe magnetic field for power transfer, the energy in a capacitivecoupling WPT system is transmitted by an alternating electricfield, as shown in Fig. 3(b). In such a system, a pair ofconductive plates are placed closely to form electrodes ofa capacitor. When an AC power supply is connected to thetransmitting plate, an alternating electric field is created. Itcauses an oscillating electric potential on the receiver platethrough the electrostatic induction and then generates AC atthe receiver side.In order to transmit energy efficiently, a high-voltage electricfield needs to be created between the electrodes of a capacitor.However, through dielectric polarization, an electric field caninteract with dielectric materials, such as the air and tissues,that are poor conductors of electricity but efficient supportersof the electrostatic field [43]. A strong electric field can also af-fect human body directly or produce noxious ozone to damagelungs [44]–[46]. Therefore, the capacitive coupling method hasmuch less commercial applications than the inductive couplingapproach for WPT.
D. Middle-Range Resonant Inductive Coupling WPT
Resonant inductive modified the inductive coupling methodto extend the distance of power transmission. In a resonantinductive coupling system, an additional LC resonance circuitthat consists of a coil and a capacitor is added at the receiverside. If the frequency of the EM field created by the sender’s Recent Qi receivers can be charged in both inductive mode and theresonant mode [38]. The inductive mode can provide a high power efficiency,while the resonant mode can increase the charging distance. oil is equivalent to the resonant frequency of LC circuit,the phase of the magnetic field is synchronized betweenthe transmitting and receiving coils, which can significantlyimprove the energy transfer efficiency. The resonant inductivecoupling method can work at medium-range, which is definedas somewhere between one and ten times of the diameter ofthe transmitting coil [47].To further extend the distance of power transmission, MITdeveloped a new technique, called WiTricity, in 2007. InWiTricity, both the sender and receiver sides have LC circuitswith the same resonant frequency, as illustrated in Fig.3(c). Asreported in [48], the energy efficiency of WiTricity is around when transmitting W of power at . MHz over thedistance up to . ft (8 times of the coil’s radius). (a) (b) Figure 5. Lighting a W bulb 6.6 feet away from an energy source throughthe WiTricity technology [49]. (a) Without obstruction. (b) A wooden panelis placed in the middle to block the direct path of the EM field.
Today, the resonant inductive coupling technology has beenwidely applied to charge mobile devices that support theresonant mode of the Qi standard [38]. It is also a promisingtechnology to enable an on-line WPT system [50], in whichthe transmitting coil is designed as a continuous power linelaid underground or multiple distributed power stations on theroadway. The power transfer initiates once a vehicle runs alongthe power line or gets close to the power station [51], [52].By using the on-line WPT technique, the battery capacity onthe vehicle can be reduced by compared to conventionalelectric cars [53]. This can greatly reduce the cost, weight,and size of vehicles.In 2013, Korea Advanced Institute of Science and Technol-ogy (KAISA) developed the world’s first WPT electric busnetwork [54], which consists of 15 miles of road in Gumi,South Korea. To power the bus, the transmitting coil is buried12 inches below the road surface, and bus chassis is 6.7 inchesfrom the ground. The total length of the power line takes upbetween 5 and 15 percent of the entire route. The frequencyof the EM field used for WPT is kHz, which can chargethe bus at kW with efficiency [55]. E. Far-Field Radiative WPT
For the far-field WPT technology, the transmission rangeof EM energy can be much longer than the size of thetransmitter. In generally, far-field WPT systems can be dividedinto two categories: high-power applications and low-powerapplications. The former have been introduced in Section I, which includes SPS and the high-density microwaves poweredaircraft; the latter will be discussed latter.A high-power WPT system usually needs a parabolic re-flector to form a narrow beam, which can significantly reducethe spreading loss of EM waves in the free space. However, anarrow beamformer needs the reflector’s aperture to be muchlarger than the wavelength. Using SPS that studied by NASAin 1979 as an example, if a satellite collects energy in thespace and then transmit it back to the earth through a . GHzmicrowave, the diameters of transmitting and the receivingreflectors need to be km and . km, respectively [56].Apparently, it is infeasible to use such a huge high-powerWPT system in a far-field WIPT device that commonly havinga portable size.In a low-power far-field WPT system, the energy is radiatedin the form of EM waves through an antenna, as demonstratedin Fig. 3(d). To maximize the energy transfer efficiency, theantenna needs to work in the resonant state. This requires thelength of the receiving antenna no less than half wavelengthof EM signals so that a stationary wave with the maximumcurrent is created in the antenna [36]. Compared with a high-power WPT system, the size of a low-power far-field WPTsystem can be small, which makes it fit the WIPT applicationsvery well. V. A RCHITECTURE OF
WIPT S
YSTEM
This section studies the implementation of WIPT, whichcombines the WPT methods discussed in Section IV with WITtechnique. In order to better understand the working principleof WIPT, we introduce the architectures of three modern WIPTsystems, they are the near-field inductively coupled WIPT, thebackscatter WIPT, and the far-field WIPT.
A. Near-Field Inductively Coupled WIPT
The inductively coupled WIPT is widely used in RFIDand NFC systems [57], [58], where the data is delivered ina passive manner. Using RFID as an example, through theinductive coupling WPT, an RF tag can receive energy froman alternating EM field created by an associated reader. Due tothe mutual inductance, an imaginary impedance will presentat the reader side once the tag gets close to it. The voltageat the reader’s antenna (coil) is proportional to the imaginaryimpedance, which decreases with the tag’s load resistance [59].Therefore, if the tag can modulate its load resistance based onthe data to be transmitted, the reader can receive that data bymeasuring the voltage variation at its antenna.Fig. 6 gives an example of how an inductively coupled RFtag sends data through the load modulation. As shown in theleft part (pink frame) of the figure, a capacitor C is connectedwith the transmitting coil in parallel to form an LC circuit.The resonant frequency of the LC circuit is equivalent to thefrequency of alternating EM field generated by the reader.When the EM signal is received by the LC circuit, it is divided Stationary wave, also called standing wave, is a combination of two wavesmoving in opposite directions, each having the same amplitude and frequency.Due to the constructive interference, the amplitude of a stationary wave is thesum of amplitudes of the two waves. oil V t t ttt t Data streamRectifierSwitch R L Comparator F r e q uenc y d i v i d e r ABC D ABCD V Switcher On/Off patternVoltage pattern at RFID reader C Structure of an RF tag Signal patterns at different points R Figure 6. An example of a near-field inductively coupled WIPT system. Theleft part is the hardware design of the tag and the right part indicates signalpatterns collected at different positions of the system. into two parts. The first part is converted into energy to powerthe tag. The second part goes through a protective resistance R to provide a frequency divider a basis signal at point Dof Fig. 6. After going through the frequency divider, a low-frequency subcarrier is available at point A. Afterward, thesubcarrier signal and the data stream enter a comparator togenerate a modulated signal. Specifically, only if the data bit is“1” (i.e., high voltage), the subcarrier signal can pass throughthe comparator; otherwise, the comparator’s output is zero, asillustrated at point C.To achieve load modulation, the comparator is connected toa field-effect transistor (FET) as a switch for an on-off control.The switch is turned ON/OFF if the output of the comparator isa positive/nonpositive value; the on-off pattern of the switchis depicted in the right part (green frame) of Fig. 6. Oncethe switch is turned on, a resistance R L is connected to thetransmitting coil in parallel. This reduces the load resistanceof the RF tag, thereby decreasing the voltage at the reader’santenna, as shown in the figure.An inductively coupled WIPT system needs to work in theinduction region of an EM field; therefore, its communicationrange is short, usually within m. Moreover, as discussed inSection IV-B, to transmit energy efficiently, the wavelength ofEM field needs to be much larger than the size of the trans-mitting coil. Therefore, the frequency of the basis signal sentfrom a reader should be low, which is commonly below kHz and . MHz for low-frequency and high-frequencyapplications, respectively [60], [61]. As a consequence, thedata rate of an inductive coupled WIPT system is low. Forexample, the data rate of NFC and RFID can only reach kbps when the basis signal of the reader is . MHz.
B. Backscatter WIPT
In a backscatter WIPT system, a reader radiates a carriersignal to an RF device, which can then modulate the intensityof a reflected carrier signal in accordance with the datastream to be transmitted. Thereafter, the reader can detect the Depending on applied coding schemes, the transmission speed of NFChas four different options: , , , or kbps [62]. amplitude fluctuation of reflected waves to decode the data.Next, we introduce two different backscatter WIPT techniques.
1) Conventional backscatter WIPT:
The idea of conven-tional backscatter WIPT is to adjust the strength of scatteredsignals. This can be achieved by changing the radar cross-section (RCS) of an antenna [63]. In backscatter WIPTsystems, a high RCS indicates that given the incident powerof EM waves, an antenna can generate a strong reflectedwave and vice versa. The total RCS of an antenna can bedivided into two components, the structural mode RCS and theantenna mode RCS. The former is determined by the antenna’sphysical feature (e.g., shape, structure, and material); the latteris caused by the antenna’s reciprocity, which radiates reflectionsignals as a transmitter depending on the operating status ofan antenna (e.g., open circuit, short circuit, or resonance).Specifically, if the impedance of a receiving antenna doesn’tmatch the circuit’s load, the incident wave will be partlyradiated back into the air [33]. Eventually, the structuralscattering field and the antenna scattering field superimposeto form the total scattered field.
Data streamRectifierMatching circuitSwitch V tt V Dipole antenna
Incident waveReflected wave M i c r o p r oce ss o r Figure 7. An example of a backscatter WIPT system, which modulates thereflected signal by switching the antenna’s status between the short circuitand the resonance for a data transmission.
The RCS is maximized when an antenna is short circuited;it is minimized when the antenna’s impedance well matchesthe circuit’s load [63]. Therefore, a backscatter WIPT systemcan modulate the amplitude of reflected waves by changingthe status of its receiving antenna for the data transmission.In Fig. 7, we provide an example of a backscatter WIPTsystem. In the figure, an FET switch is connected with a dipoleantenna in parallel. The ON/OFF of the switch is manipulatedby the data sequence to be transmitted. When the switch isON, the antenna is short-circuited to maximize RCS, therebycausing a strong backscattered wave. When the switch is OFF,the matching circuit makes the impedances of the antennaand the circuit load identical to minimize the backscatteringstrength. Through measuring the amplitude of reflected waves,the reader can decode the information from the sender.To increase the data rate of a backscatter WIPT system,we can adopt a quadrature amplitude modulation (QAM) atthe sender side [64]. This can be implemented by connectingthe transmitting antenna with multiple different resistors inparallel, as shown in Fig. 8. During communication, L -bitsdata sequence turns on one out of n switches and then theconnected resistor creates a certain level of load mismatch,where L = log ( n ) . Accordingly, the reflected waves can have different strengths, and each strength can represent L -bitsdata. D i p o l e an t enna … S S S S n R n R R R Figure 8. Frontend of a backscatter WIPT circuit to generate reflected QAMsignals, where S i and R i represent the i th FET switch and the i th resistor,respectively. The communication range and the data rate of a conven-tional backscatter WIPT system are mainly determined by theradiation power of the reader and the size of the transmittingantenna. Owing to the high carrier frequency, the transmissionrate of a backscatter WIPT system can reach Mbps oreven higher, much faster than that in an inductively coupledWIPT system [65]. The communication rage of a conventionalbackscatter WIPT system is usually within m due to a highspreading loss of scattering signals and an imperfect reflectionof the transmitting antenna [66]. As measured in [67], if areader uses mW power to transmit a MHz carrier signalvia a dBi antenna and an RF tag is placed m away from thereader to reflect the carrier signal through a mm × mmpatch antenna, then the strengths of reflected signals measuredby the reader are only − . dBm and − . dBm when thetag’s antenna is in the short circuit mode and the load matchmode, respectively.
2) Ambient backscatter WIPT:
Recently, a new backscatterWIPT system, called the ambient backscatter assisted com-munication, is proposed [68]. Instead of using the RF wavetransmitted from a dedicated reader as the carrier signal andenergy medium, the new system harvests and reflects ambientradio waves radiated from TV towers and cellular base stationsfor wireless communication.In the ambient backscatter WIPT system, a sender performsthe load modulation to change the amplitude of waves reflectedfrom ambient RF. The receiver can be another backscatteringnode [24] or a small RF transmitter, e.g., a wireless router [69].The received data can be decoded by measuring the averagestrength of backscattering waves arrived in a certain period oftime.To apply the ambient backscatter WIPT in the real world,one challenge is the irregular carrier problem. Specifically,an ideal carrier signal is a CW wave, whose amplitude andfrequency are constant. However, most ambient RF signals area combination of modulated signals, their amplitude and fre-quency change with time. As a result, the random fluctuationin amplitudes of the ambient carrier signal may overwhelmthe change in intensities of the reflected wave caused by thesender’s load modulation. To detect the latter reliably, thetransmitter needs to increase the energy per bit by reducing the data rate so that the accumulative energy of reflected wavescan have a noticeable difference when different data bits aresent [24], [70]. As a consequence, the ambient backscatterWIPT systems usually have a low transmission rate and ashort communication range.
C. Far-Field WIPT
Different from the inductively coupled WIPT and thebackscatter WIPT, the far-field WIPT can transmit data ac-tively without any incoming signal as the information carrier.Depending on whether or not the energy harvesting circuit andthe communication module share the antenna, far-field WIPTsystems can be divided into the following three categories:
1) Separated system:
In a separated far-field WIPT system,the energy harvester has a dedicated antenna and thereforecan be considered as a battery, which replenishes energy au-tomatically and supplies power to the wireless communicationmodule.
2) Co-located system:
In a co-located far-field WIPT sys-tem, the energy harvester and the communication moduleshare an antenna [29], as shown in Fig. 9. According to howthe energy harvesting process coordinates with wireless com-munications, a co-located WIPT system has three branches,which are the time-switching, power-splitting, and antennaswitching systems. In the time-switching WIPT, a systemcan either harvest energy or communicate with other wirelessdevices, but not at the same time. In the power-splitting WIPT,the incident RF wave is split into two streams by a powerdivider. The stream went through the communication moduleis decoded as an information; the stream flowed through theenergy harvester is converted into energy to power the wholesystem. In the antenna switching WIPT, multiple receivingantennas are divided into two groups and each group isarranged to send/receive data or harvest energy.
3) Integrated system:
In an integrated WIPT system, therectifier in the energy harvesting circuit is also used as a low-pass filter to extract the baseband signal from received RFsignals [71]. After that, the baseband signal is split into anenergy stream and a data stream by a power switcher or atime switcher in order to receive energy and information atthe same time.Compared with a co-located system or an integrated WIPTsystem, the separated architecture has two advantages. First,it can optimize the energy harvesting efficiency and the datarate simultaneously in a far-field RF environment. To achievethis, separated antennas are necessary since the energy harvestand the wireless communications need antennas with differentcharacteristics [72]. Specifically, to harvest energy efficiently,the antenna should have a ultra-wide frequency band or mul-tiple resonant frequencies to collect energy from a wide rangeof frequencies, while for wireless communications, an antennawith a moderate bandwidth is preferred in order to maximizethe antenna gain on a certain frequency while reducing thewide-band noise [73], [74]. Secondly, in a separated WIPTsystem, the energy stream and the data stream can havedifferent frequencies, thereby preventing the RF energy frominterfering with information decoding. ase stationAntenna Rectifier & voltage multiplier B a tt e r y Matching circuitRF signal Power divider Amplifier Low pass filter A/D converter DSPD/A converterEnergy flow Data flow (reception)Demodulator ModulatorBand-pass filterData flow (transmission)
Figure 9. An example of a far-field WIPT system, where the blue frameand the green frame are the energy harvesting circuit and the communicationmodule, respectively.
In a co-located system, the energy harvester and the com-munication module share an antenna to reduce the systemsize. However, sharing the antenna usually requires the energystream and the information stream to have the same frequencyband. Hence, a power divider in the co-located WIPT needs tocarefully adjust the distribution ratio between the energy flowand the data flow so that the system can have enough signal-to-noise ratio (SNR) to decode information and sufficient powerto drive the system. R
EFERENCES[1] G. Goubau and F. Schwering, “On the guided propagation of electro-magnetic wave beams,”
IRE Transactions on Antennas and Propagation ,vol. 9, no. 3, pp. 248–256, 1961.[2] J. E. Degenford, M. Sirkis, and W. Steier, “The reflecting beam waveg-uide,”
IEEE Transactions on Microwave Theory and Techniques , vol. 12,no. 4, pp. 445–453, 1964.[3] R. George and E. Sabbagh, “An efficient means of converting microwaveenergy to dc using semiconductor diodes,”
Proceedings of the IEEE ,vol. 51, no. 3, pp. 530–530, 1963.[4] W. Brown, “Experiments in the transportation of energy by microwavebeam,” in , vol. 12. IEEE,1966, pp. 8–17.[5] J. J. Schlesak, A. Alden, and T. Ohno, “A microwave powered highaltitude platform,” in
Proceedings of the IEEE MTT InternationalMicrowave Symposium Digest (MTT) . IEEE, 1988, pp. 283–286.[6] P. E. Glaser, “Power from the sun: Its future,”
Science , vol. 162, no.3856, pp. 857–861, 1968.[7] R. Dietz, G. Arndt, J. Seyl, L. Leopold, and J. Kelly, “Satellite powersystem: Concept development and evaluation program,”
NASA ReferencePublication , vol. 1076, 1981.[8] S. Sasaki, K. Tanaka, and K.-i. Maki, “Microwave power transmissiontechnologies for solar power satellites,”
Proceedings of the IEEE , vol.101, no. 6, pp. 1438–1447, 2013.[9] C. Shannon, “A mathematical theory of communication,”
The BellSystem Technical Journal , vol. 27, no. 3, pp. 379–423, 1948.[10] A. A. Huurdeman,
The worldwide history of telecommunications . JohnWiley & Sons, 2003.[11] M. Vidojevic, “2G/3G/4G — is it all about the speed,” mikroe.com,2016, [Accessed: November, 2017]. [Online]. Available: https://learn.mikroe.com/2g-3g-4g-speed/[12] J. G. Andrews, S. Buzzi, W. Choi, S. V. Hanly, A. Lozano, A. C. Soong,and J. C. Zhang, “What will 5G be?”
IEEE Journal on selected areasin communications , vol. 32, no. 6, pp. 1065–1082, 2014.[13] W. C. Brown, “The history of power transmission by radio waves,”
IEEETransactions on microwave theory and techniques , vol. 32, no. 9, pp.1230–1242, 1984.[14] H. Stockman, “Communication by means of reflected power,”
Proceed-ings of the IRE , vol. 36, no. 10, pp. 1196–1204, 1948. [15] A. R. Koelle, S. W. Depp, and R. W. Freyman, “Short-range radio-telemetry for electronic identification, using modulated RF backscatter,”
Proceedings of the IEEE , vol. 63, no. 8, pp. 1260–1261, 1975.[16] R. Want, “Near field communication,”
IEEE Pervasive Computing ,vol. 10, no. 3, pp. 4–7, 2011.[17] J. Landt, “The history of RFID,”
IEEE potentials , vol. 24, no. 4, pp.8–11, 2005.[18] J. Singh, N. Brar, and C. Fong, “The state of RFID applications inlibraries,”
Information technology and libraries
IEEE Annalsof the History of Computing
Proceedings of the IEEE , vol. 94, no. 6, pp. 1177–1196, 2006.[24] V. Liu, A. Parks, V. Talla, S. Gollakota, D. Wetherall, and J. R.Smith, “Ambient backscatter: wireless communication out of thin air,”
SIGCOMM Computer Communication Review Proceedings of IEEE International Symposium on Information Theory(ISIT) . IEEE, 2008, pp. 1612–1616.[29] R. Zhang and C. K. Ho, “MIMO broadcasting for simultaneous wire-less information and power transfer,”
IEEE Transactions on WirelessCommunications , vol. 12, no. 5, pp. 1989–2001, 2013.[30] Q. Wu, G. Y. Li, W. Chen, D. W. K. Ng, and R. Schober, “An overviewof sustainable green 5G networks,”
IEEE Wireless Communications ,vol. 24, no. 4, pp. 72–80, 2017.[31] Y. Liu, Y. Zhang, R. Yu, and S. Xie, “Integrated energy and spectrumharvesting for 5G wireless communications,”
IEEE Network , vol. 29,no. 3, pp. 75–81, 2015.[32] A. B. Constantine et al. , Antenna theory: analysis and design (fourthedition) . John Wiley & Sons, 2015.[33] W. L. Stutzman and G. A. Thiele,
Antenna theory and design (thirdedition) . John Wiley & Sons, 2012.[34] A. Umenei, “Understanding low frequency non-radiative power transfer,”
Fulton Innovation , 2011.[35] M. N. Sadiku,
Elements of electromagnetics (sixth edition) . Oxforduniversity press, 2014.[36] G. Hall,
The ARRL antenna book (thirteenth edition) . The AmericanRadio Relay League, Inc., 1980.[37] G. S. Smith,
An introduction to classical electromagnetic radiation
IEEE Electrification Magazine ,vol. 1, no. 1, pp. 57–64, 2013.[43] S. Gabriel, R. Lau, and C. Gabriel, “The dielectric properties ofbiological tissues: II. measurements in the frequency range 10 Hz to20 GHz,”
Physics in medicine and biology , vol. 41, no. 11, p. 2251,1996.[44] B. Eliasson, M. Hirth, and U. Kogelschatz, “Ozone synthesis fromoxygen in dielectric barrier discharges,”
Journal of Physics D: AppliedPhysics , vol. 20, no. 11, p. 1421, 1987.[45] R. B. Devlin, W. F. McDonnell, R. Mann, S. Becker, D. E. House,D. Schreinemachers, and H. S. Koren, “Exposure of humans to ambientlevels of ozone for 6.6 hours causes cellular and biochemical changesin the lung,”
Am J Respir Cell Mol Biol , vol. 4, no. 1, pp. 72–81, 1991.[46] C. Gabriel, S. Gabriel, and E. Corthout, “The dielectric properties ofbiological tissues: I. Literature survey,”
Physics in medicine and biology ,vol. 41, no. 11, p. 2231, 1996.[47] J. I. Agbinya,
Wireless power transfer . River Publishers, 2015, vol. 45.[48] A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, andM. Soljaˇci´c, “Wireless power transfer via strongly coupled magneticresonances,” science
Proceedings ofthe IEEE Energy Conversion Congress and Exposition (ECCE) . IEEE,2010, pp. 1598–1601.[51] J. M. Miller, O. C. Onar, C. White, S. Campbell, C. Coomer, L. Seiber,R. Sepe, and A. Steyerl, “Demonstrating dynamic wireless charging ofan electric vehicle: the benefit of electrochemical capacitor smoothing,”
IEEE Power Electronics Magazine , vol. 1, no. 1, pp. 12–24, 2014.[52] F. Musavi, M. Edington, and W. Eberle, “Wireless power transfer: asurvey of ev battery charging technologies,” in
Proceedings of the IEEEEnergy Conversion Congress and Exposition (ECCE) . IEEE, 2012, pp.1804–1810.[53] S. Ahn and J. Kim, “Magnetic field design for high efficient and lowEMF wireless power transfer in on-line electric vehicle,” in
Proceedingsof the European Conference on Antennas and Propagation (EUCAP)
Proceedings of IEEE Conference Record of World Conference onPhotovoltaic Energy Conversion , vol. 2. IEEE, 2006, pp. 1939–1942.[57] R. Weinstein, “RFID: a technical overview and its application to theenterprise,”
IT professional , vol. 7, no. 3, pp. 27–33, 2005.[58] V. Coskun, B. Ozdenizci, and K. Ok, “The survey on near fieldcommunication,”
Sensors , vol. 15, no. 6, pp. 13 348–13 405, 2015.[59] K. Finkenzeller,
RFID handbook: fundamentals and applications incontactless smart cards, radio frequency identification and near-fieldcommunication
Radar cross section measurements . Springer Science &Business Media, 2012.[64] S. Thomas and M. S. Reynolds, “QAM backscatter for passive UHFRFID tags,” in
Proceedings of the IEEE International Conference OnRFID
IEE Proceedings-Microwaves, Antennas and Propagation , vol. 153, no. 1, pp. 103–109,2006.[68] X. Lu, D. Niyato, H. Jiang, D. I. Kim, Y. Xiao, and Z. Han, “Ambientbackscatter assisted wireless powered communications,”
IEEE WirelessCommunications , 2018.[69] D. Bharadia, K. R. Joshi, M. Kotaru, and S. Katti, “Backfi: highthroughput wifi backscatter,”
ACM SIGCOMM Computer Communica-tion Review , vol. 45, no. 4, pp. 283–296, 2015.[70] A. N. Parks, A. Liu, S. Gollakota, and J. R. Smith, “Turbochargingambient backscatter communication,”
ACM SIGCOMM Computer Com-munication Review , vol. 44, no. 4, pp. 619–630, 2015.[71] X. Zhou, R. Zhang, and C. K. Ho, “Wireless information and powertransfer: Architecture design and rate-energy tradeoff,”
IEEE Transac-tions on communications , vol. 61, no. 11, pp. 4754–4767, 2013.[72] L.-G. Tran, H.-K. Cha, and W.-T. Park, “RF power harvesting: a reviewon designing methodologies and applications,”
Micro and Nano SystemsLetters , vol. 5, no. 1, p. 14, 2017.[73] C. Song, Y. Huang, J. Zhou, J. Zhang, S. Yuan, and P. Carter, “A high-efficiency broadband rectenna for ambient wireless energy harvesting,”
IEEE Transactions on Antennas and Propagation , vol. 63, no. 8, pp.3486–3495, 2015.[74] Y.-H. Suh and K. Chang, “A high-efficiency dual-frequency rectenna for2.45- and 5.8-GHz wireless power transmission,”