Aug.2019Vol.36No.4 Transactions of Nanjing University of Aeronautics and Astronautics
Retro‑reflective Beamforming Technique with Applications in
hugh hefnerWireless Power Transmission
WANG Xin1*，LU Mingyu2
syracu university1.College of Electronic and Information Engineering，Nanjing University of Aeronautics and Astronautics，
wheel alignmentNanjing211106，P.R.China；
2.Department of Electrical and Computer Engineering，West Virginia University Institute of Technology，
West Virginia25801，USA
（Received5July2019；revid25July2019；accepted30July2019）
vnnAbstract:Retro⁃reflective beamforming technique has the potential of enabling efficient wireless power t
ransmission over long distance（on the order of meters and even kilometers）.In retro⁃reflective beamforming，wireless power transmission is guided by pilot signal：Bad upon pilot signal broadcasted by a wireless power receiver，a wireless power transmitter delivers focud microwave power beam（s）onto the location of wireless power receiver.When the wireless power receiver’s location is not fixed or when the wireless power receiver’s location is unknown to the wireless power transmitter，the microwave power beam would follow the wireless power receiver’s location dynamically as long as the wireless power receiver broadcasts pilot signal periodically.This paper reviews our rearch endeavors in recent years on retro⁃reflective beamforming technique targeting three applications：（1）wireless charging for low⁃power mobile/portable electronic devices，（2）space solar power satellites（SSPS）application，and （3）wireless charging in fully⁃enclod space.The feasibility and potential of retro⁃reflective beamforming technique with applications in wireless power transmission are demonstrated by some preliminary experimental results.
the ringKey words:wireless power transmission;retro⁃reflective beamforming;microwave;antenna array;pilot signallpk
CLC number：TN99Document code：A Article ID：1005⁃1120（2019）04⁃0545⁃21
0Introduction
Wireless power transmission，i.e.，delivering electrical power without using wires/cables，implies numerous appealing applications.The best⁃known wireless power transmission technique is inductive coupling［1］；for instance，commercially⁃available wi⁃reless charging pads for cell phones are bad on in⁃ductive coupling technique.As the inductive cou⁃pling technique takes advantage of non⁃propagating magnetic field，wireless power transmission bad on inductive coupling is limited to short distance（on the order of centimeters，typically）.Employing propagating electromagnetic waves in the radio fre⁃quency（RF）or microwave frequency range as the carrier of wireless power has been demonstrated ca⁃pable of reaching long distance（on the order of me⁃ters and even kilometers）［2］.Meanwhile，optical po⁃wer transmission（that is，employing optical waves as the carrier of wireless power）is also capable of reaching long distance［3⁃4］.Relative to optical power transmission，RF/microwave power transmission enjoys veral advantages.First，RF/microwave has better penetration capability than optical waves. Second，conversion efficiency between DC power and microwave power is usually higher than that be⁃tween DC power and optical power.Third，micro⁃wave beams can be steered straightforwardly via pha control，whereas beam steering without re⁃sorting to mechanical motion is much more difficult in the optical regime.Thus，RF/microwave power transmission is an excellent candidate when the dis⁃
∗Corresponding author，E⁃mail address:wang90@nuaa.edu.
How to cite this article:WANG Xin，LU Mingyu.Retro⁃reflective Beamforming Technique with Applications in Wireless Power Transmission［J］.Transactions of Nanjing University of Aeronautics and Astronautics，2019，36（4）：545⁃565.
http：//dx.doi/10.16356/j.1005⁃1120.2019.04.002
Vol.36
Transactions of Nanjing University of Aeronautics and Astronautics tance between wireless power transmitter and wire⁃less power receiver is beyond the order of centime⁃ter and when the spatial relationship between wire⁃less power transmitter and wireless power receiver is not fixed.
As a far⁃from⁃complete review of the history of RF/microwave power transmission ，veral histori⁃cal experiments are highlighted below.In 1960s ，Brown demonstrated supplying microwave power from a ground station to a helicopter ，which is prob⁃ably the first impactful and well⁃documented demon⁃stration of RF/microwave power transmission in the history ［5］.In an experiment carried out by NASA JPL in 1975，30kW of RF power was successfully delivered over one mile ，i.e.，1.6km ［6⁃7］.The Sta⁃tionary High Altitude Relay Program （SHARP ）ini⁃tiated in Canada in 1980s aims to provide micro⁃wave power to small aircrafts ［8］.A program similar to
SHARP ，named MIcrowave Lifted Airplane eX⁃periment （MILAX ），was active in Japan in 1990s ［9］.In 1993，International Space Year⁃Micro⁃wave Energy Transmission （ISY⁃METS ）experi⁃ments were conducted in Japan to achieve micro⁃wave power transmission between spacecrafts ［10］.A ca study from 1997to 2004is reported in Ref.［11］to construct point⁃to⁃point wireless electricity transmission to a small isolated village called Grand⁃Bassin in France.In 2009，the feasibility of using a car⁃borne power broadcaster to power n⁃sors installed over a bridge is studied in Ref.［12］.2013湖南高考英语
Since 2013，a range of rearch efforts on mi⁃crowave power transmission are conducted collabor⁃atively between Nanjing University of Aeronautics and Astronautics and West Virginia University ［13］.Specifically ，the collaborative rearch focus on retro⁃reflective beamforming technique ，as depicted
by Fig.1.The wireless power transmitter includes an array of antenna elements.A wireless power re⁃ceiver receives wireless power from the wireless power transmitter via the following two steps ：
（1）The wireless power receiver broadcasts pi⁃lot signal ，which is a low⁃power signal.
（2）In respon to the pilot signal ，the wireless power transmitter constructs focud microwave
英语作文 我的梦想
power beam onto the location of wireless power re⁃ceiver.
shine your wayWhen the wireless power receiver is in motion ，the microwave power beam would follow the wire⁃less power receiver ’s location dynamically as long as the wireless power receiver broadcasts pilot sig⁃nal periodically.
The underlying theory of retro⁃reflective beam ⁃forming is “time⁃reversal ”，which takes advantage of channel reciprocity to accomplish a space⁃time matched filter ［14］.Specifically ，the propagation of pi⁃lot signal follows “the channel from wireless power receiver to wireless power transmitter ”，whereas the propagation of microwave power follows “the channel from wireless power transmitter to wireless power receiver ”.If the two channels are reciprocal to each other and if the microwave power transmis⁃sion is tailored to be the retro⁃reflected version of pi⁃lot signal ，microwave power is spatially focud on⁃to the location from which the pilot signal stems ，that is ，the location of the wireless power receiver.Furthermore ，spatial focusing due to retro⁃reflec⁃tion/time⁃reversal does not suffer from multi⁃path in environments ［15⁃16］.As another magnificent advan⁃tage of retro⁃reflective beamforming technique ，re⁃configuration of microwave power transmission could be accomplished electronically ，i.e.，without resorting to mechanical
control.
Fig.1Depiction of retro⁃reflective beamforming technique for wireless power transmission
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No.4WANG Xin,et al.Retro⁃reflective Beamforming Technique with Applications in Wireless…
Retro⁃reflective beamforming technique has been rearched for years［17⁃19］，with widespread ap⁃plications including wireless communication，radar，and wireless power transmission.It is worth
while noting that，retro⁃reflective beamforming in the con⁃text of wireless power transmission application has two unique technical concerns compared with wire⁃less communication and radar applications.First，the wireless power transmission application aims at maximal power transmission efficiency.Typically in order to achieve high power transmission efficiency，the transmitter and/or the receiver must employ an⁃tennas with large aperture.When the antenna dimen⁃sion is large with respect to the transmitter⁃receiver distance，the far⁃zone condition（that is，d>2D2/λ，where d is the distance between trans⁃mitter and receiver，D the antenna dimension，andλthe wavelength）would not be satisfied.As a result，the transmitter and receiver stay in each other’s near⁃zone rather than far⁃zone in a typical wireless power transmission application.Second，in the wire⁃less power transmission application the power level associated with wireless power propagation（Fig.1（b））is much higher than the power level associated with pilot signal propagation（Fig.1（a））.Con⁃quently，it is vital to isolate pilot signal propagation from wireless power propagation，becau the wire⁃less power propagation is considered as an interfer⁃ence/jammer by the receiver of pilot signal.
Fig.2illustrates an implementation scheme of retro⁃reflective beamforming，with the assumption that the transmitter and receiver are in each other’s far⁃zone，in other words，with the assumption that d>2D2/λis satisfied.Also，the antenna elements in the wireless power transmitter are assumed to be
equi⁃spaced along one dimension in space.In the first step of retro⁃reflective beamforming（Fig.2（a）），the wireless power receiver broadcasts a con⁃tinuous⁃wave pilot signal.Since the wireless power receiver resides in the far⁃zone of wireless power transmitter，pilot signal reaches the wireless power transmitter as a plane wave with planar equi⁃pha surfaces，as depicted in Fig.2（a）.As a result，the phas detected by the array’s elements exhibit a lin⁃ear pattern.In the cond step of retro⁃reflective beamforming（Fig.2（b）），the wireless power trans⁃mitter transmits wireless power，which is a continu⁃ous⁃wave with the same frequency as pilot signal’s （but of higher power level），to the wireless power receiver.Each antenna element is excited with pha negative to the pha of pilot signal received in the first step，as shown in Fig.2（b）.In other words，the pha profile of wireless power transmis⁃sion in the cond step is conjugate to the pha pro⁃file of pilot signal reception in the first step.Accord⁃ing to the theory of phad array，the wireless pow⁃er transmitter generates a plane wave with propaga⁃tion direction opposite to the pilot signal’s incoming direction.The scheme illustrated in Fig.2is termed “retro⁃directive beamforming”in many literatures，since pilot signal propagation and wireless power propagation both exhibit explicit propagation direc⁃tions.As a matter of fact，retro⁃directive beamform⁃ing is a special ca of retro⁃reflective beamforming when the wireless power receiver resides in the far⁃zone of wireless power transmitter.
If a wireless power receiver does not reside in the far⁃zone of wireless power transmitter，the im⁃plementation scheme of retro⁃reflective beamform⁃ing is esntially the same as Fig.2，but with
some Fig.2An implementation scheme of retro⁃reflective beamforming when a wireless power receiver resides in the far⁃zone of wireless power transmitter
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Transactions of Nanjing University of Aeronautics and Astronautics slight and interesting differences ，as illustrated in Fig.3.In the first step ，the pilot signal does not be⁃have as a plane wave when it reaches the wireless power transmitter ；rather ，its equi⁃pha surfaces are curved ，as displayed by Fig.3（a ）.Conquent⁃ly ，the pha profile of pilot signal received by the antenna elements does not exhibit linear pattern.When the pha received by the right⁃most element is defined to be zero ，the pha received by an ele⁃ment in the middle is denoted as -θ；apparently ，pha -θcorresponds to the extra path specified by a piece of thick line gment in Fig.3（a ）.If the right⁃most element is excited with pha zero and the middle element is excited with pha θin the c⁃
ond step （Fig.3（b ）），their radiations reach the wire⁃less power receiver with the same pha ，in other words ，their radiations are constructive at the wire⁃less power receiver.The above analysis holds true for every element of the array.Therefore ，exciting the array elements with pha profile conjugate to the pilot signal ’s pha profile leads to focusing wireless power onto the wireless power receiver ’s location.When the wireless power receiver moves further and further away from the wireless power transmitter ，the scenario in Fig.3evolves to Fig.2.Thus ，Fig.2is a special ca of Fig.3，and Fig.3embodies a general scheme of retro⁃reflective beam ⁃forming technique.
Figs.2and 3jointly explain the relationship be⁃tween “retro⁃reflective beamforming ”and “retro⁃di⁃rective beamforming ”.Specifically ，“retro⁃directive beamforming ”is a special ca of “retro⁃reflective beamforming ”when the wireless power receiver re⁃sides in the far⁃zone of wireless power transmitter ，whereas “retro⁃reflective beamforming ”is applica⁃ble in both far⁃zone and near⁃zone scenarios ［20］.In a typical wireless power transmission application ，the transmitter and receiver do not stay in each other ’s far⁃zone ，as discusd earlier in this ction.Hence in our endeavors ，“retro⁃reflective beamforming ”is ud as the name of the technique we pursue to ac⁃complish efficient wireless power transmission ，al⁃beit “retro⁃directive beamforming ”is adopted in lit⁃eratures more popularly.
Numerous technical approaches have been pro⁃pod to implement the retro⁃reflective beamform ⁃ing schemes illustrated in Fig.2or Fig.3，such as Van Atta method ［21］，heterodyne method ［22］，and methods bad on pha⁃lock loop ［23］.Nevertheless ，
some of the methods cannot be applied to wireless power transmission applications directly.For exam ⁃ple in wireless power transmission applications ，it is vital to prevent wireless power from jamming the re⁃ceiver of pilot signal ；and thus if a method does not offer sufficient isolation between wireless power and pilot signal ，it would not be a good option for wire⁃less power transmission applications.Moreover in practice ，various wireless power transmission appli⁃cations are drastically different from one another （three of them are elucidated in this paper ）.It is al⁃most impossible for one technical approach to fit all the applications.During the past few years ，we con⁃ducted a range of rearch on retro⁃reflective beam ⁃forming targeting three specific wireless power transmission applications ：（1）wireless charging for low⁃power mobile/portable electronic devices ，（2）space solar power satellites （SSPS ）application ，and （3）wireless charging in fully⁃enclod space.Our rearch efforts and findings for the three ap⁃plications are prented in Sections 1，2，and 3，
re⁃
Fig.3An implementation scheme of retro⁃reflective beamforming when a wireless power receiver resides in the near⁃zone of wireless power transmitter
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spectively.The last ction of this paper，Section 4，relates to our conclusions regarding retro⁃reflec⁃tive beamforming technique with applications in wireless power transmission.
1Retro‑reflective Beamforming Technique for Charging Low‑
Power Mobile/Portable Electron‑
ic Devices Wirelessly
Numerous portable electronic devices，such as laptops，cell phones，digital cameras，and electric shavers，rely on rechargeable batteries and must be routinely charged by the line power.A wireless charging technique capable of delivering electrical power to the portable devices would make them tether free and“truly portable”.Nevertheless，a range of established regulations impos
restrictions on the power level associated with wireless trans⁃mission，for the purpo of electromagnetic compati⁃bility and human safety［24⁃26］.Compliance with the regulations makes it a challenging task to deliver wireless power of a few Watts or higher remotely. Remote delivery of wireless power on the order of milli⁃Watt，though not sounding extremely excit⁃ing，is still uful in practice.In fact，a large num⁃ber of low⁃power electronic devices like radio fre⁃quency identification tags and wireless nsors may benefit from wireless power transmission，particu⁃larly in scenarios where wired charging is intractable （for examples，when tags are unattended and when nsors are buried underneath the ground）［27］.
海上钢琴师中英双字In Fig.4（a），a warehou is ud as an exam⁃ple of practical environments in which“wireless charging for low⁃power mobile/portable electronic devices”could find applications.Suppo there are thousands of containers in the warehou，and each container has one radio frequency identification tag attached to it.Fig.4（b）depicts a retro⁃reflective beamforming scheme to deliver wireless power to the tags.The wireless power transmitter consists of a ba station and multiple charging panels.The charging panels are mounted over the ceiling or walls.The ba station and charging panels are con⁃nected to each other through cables.Tags in the warehou receive wireless power from the wireless power transmitter via the following two steps：（1）One or more t
han one tag（s）broadcast pilot sig⁃nals；（2）In respon to the pilot signals，the charg⁃ing panels jointly construct focud microwave pow⁃er beam（s）onto the target tag（s）.
A charging panel transmits power only if it has line⁃of⁃sight interaction with the target tag.If the line⁃of⁃sight path is blocked by any obstacle，the charging panel is deactivated such that the obstacle，which might be human being，is not illuminated by power beams directly［28］.
Timing quence of the retro⁃reflective beam⁃forming scheme in Fig.4is depicted by a flow chart in Fig.5.Interactions between the wireless power transmitter and wireless power receiver are toggled among three modes：communication mode，radar mode，and charging mode.The process in Fig.
5 Fig.4An example of wireless charging for low⁃power mobile/portable electronic devices
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