CN114039680B - Method for measuring strength of backscatter signal - Google Patents
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- CN114039680B CN114039680B CN202111179688.0A CN202111179688A CN114039680B CN 114039680 B CN114039680 B CN 114039680B CN 202111179688 A CN202111179688 A CN 202111179688A CN 114039680 B CN114039680 B CN 114039680B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B17/00—Monitoring; Testing
- H04B17/30—Monitoring; Testing of propagation channels
- H04B17/309—Measuring or estimating channel quality parameters
- H04B17/318—Received signal strength
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
- G06K17/00—Methods or arrangements for effecting co-operative working between equipments covered by two or more of main groups G06K1/00 - G06K15/00, e.g. automatic card files incorporating conveying and reading operations
- G06K17/0022—Methods or arrangements for effecting co-operative working between equipments covered by two or more of main groups G06K1/00 - G06K15/00, e.g. automatic card files incorporating conveying and reading operations arrangements or provisious for transferring data to distant stations, e.g. from a sensing device
- G06K17/0029—Methods or arrangements for effecting co-operative working between equipments covered by two or more of main groups G06K1/00 - G06K15/00, e.g. automatic card files incorporating conveying and reading operations arrangements or provisious for transferring data to distant stations, e.g. from a sensing device the arrangement being specially adapted for wireless interrogation of grouped or bundled articles tagged with wireless record carriers
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- G06—COMPUTING; CALCULATING OR COUNTING
- G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
- G06K19/00—Record carriers for use with machines and with at least a part designed to carry digital markings
- G06K19/06—Record carriers for use with machines and with at least a part designed to carry digital markings characterised by the kind of the digital marking, e.g. shape, nature, code
- G06K19/067—Record carriers with conductive marks, printed circuits or semiconductor circuit elements, e.g. credit or identity cards also with resonating or responding marks without active components
- G06K19/07—Record carriers with conductive marks, printed circuits or semiconductor circuit elements, e.g. credit or identity cards also with resonating or responding marks without active components with integrated circuit chips
- G06K19/077—Constructional details, e.g. mounting of circuits in the carrier
- G06K19/07749—Constructional details, e.g. mounting of circuits in the carrier the record carrier being capable of non-contact communication, e.g. constructional details of the antenna of a non-contact smart card
- G06K19/07758—Constructional details, e.g. mounting of circuits in the carrier the record carrier being capable of non-contact communication, e.g. constructional details of the antenna of a non-contact smart card arrangements for adhering the record carrier to further objects or living beings, functioning as an identification tag
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Abstract
The invention relates to a method for measuring the strength of a backscattering signal, which comprises the following steps: the method comprises the following steps: the reader-writer sends a continuous carrier signal to the label; step two: the label receives the continuous carrier signal, and controls the backscattering of the continuous carrier signal according to the modulation signal p (t) of the label to obtain a label backscattering signal; step three: the reader-writer receives the tag backscatter signal and measures the frequency spectrum of the tag backscatter signal; step four: and according to the frequency spectrum measurement result of the reader-writer, the amplitude of the tag backscatter signal is solved. By adopting the method provided by the invention, the strength of the label backscatter signal can be accurately obtained through simple measurement and theoretical calculation, the implementation is simple, the calculated amount is small, and the method has important significance for analyzing the performance of the RFID reader-writer.
Description
Technical Field
The invention relates to the technical field of Internet of things and backscatter communication, in particular to a method for measuring the strength of a backscatter signal.
Background
Backscattering (Backscatter) refers to reflecting a wave or signal back from an incident direction. The backscattering technique has a wide and mature application in Radio Frequency Identification (RFID) systems and radar systems.
A Reader-writer (Reader) in the radio frequency identification system utilizes a backscattering technology to perform information interaction with a Tag (Tag) in a radio frequency signal space coupling mode, so that non-contact information identification is realized. The method is widely applied to the fields of non-contact identification cards, electronic toll collection systems, identity identification, access control systems, warehouse management and the like.
Ultra High Frequency (UHF) RFID systems have received much attention due to the advantages of long read/write distances and high identification speeds. The structure of the UHF RFID system is shown in figure 1 and mainly comprises a reader-writer, an antenna and a label. The reader-writer is the core of the system and continuously sends continuous Carrier (CW) signals to provide energy for the label; the CW signal is radiated into a free space through an antenna; the Tag receives the CW signal sent by the Reader and backscatters the CW signal to transmit the signal to the Reader (the Tag signal reflects CW for "1", and the Tag signal does not reflect CW for "0"). This means that the Reader receives the signal from the Tag and simultaneously transmits the CW signal through its own TX port, which will couple to the RX port as a self-interference signal, which causes severe interference to the useful signal received by the RX port, thereby affecting the receiving sensitivity of the Reader.
In general, the Tag backscattered useful signal is much smaller than the self-interference signal due to free-space propagation loss. The relation between the distance between the Tag and the Reader and the signal power returned by the Reader receiving the Tag is obtained through simulation in the document [1], and if the isolation of the circulator is 30dB and the power of a CW signal transmitted by the Reader is 33dBm, the power of a self-interference signal coupled to an RX port is 3dBm, and when the distance is 1.6m-40m, the power of the signal returned by the Reader receiving the Tag is smaller than-45 dBm. In this case, the self-interference signal power is 48dB stronger than the useful signal power.
The Reader converts an analog signal into a digital signal generally through four steps: sampling, holding, quantizing, and encoding are typically performed by Analog-to-Digital Conversion (ADC) chips. The commonly used ADCs have 8 bits, 10 bits, 12 bits, 14 bits and 16 bits, and the higher the accuracy of the ADCs is, the resources inside the device are greatly increased, the cost and price of the device are also greatly increased, and the price of the 12 bits ADCs is twice higher than that of the 10 bits ADCs under the condition of the same sampling rate. Generally, the number of bits of the ADC is doubled by the price of two bits. Therefore, the Reader ADC is selected to integrate factors such as precision and price, and taking a 14-bit ideal ADC as an example, the dynamic range of a signal to be resolved is 42dB. This also means that for the ADC to quantize the Tag backscattered signal, it should be ensured that the ratio of the amplitude of the self-interference signal to the useful signal is less than 42dB, and the smaller the ratio of the amplitudes of the two is, the more beneficial the ADC quantization. Obviously, the strength of the Tag useful signal is accurately measured, and the performance of the UHF RFID reader-writer is favorably analyzed.
However, the signal received at the Reader RX port is an alias of the self-interference signal and the Tag backscattered signal, and the Tag backscattered useful signal is much smaller than the self-interference signal, so that the Tag backscattered signal strength cannot be directly measured. The method can meet the measurement of the strength of the UHF RFID label backscatter signal in different application scenes, has simple measurement method and has important significance for analyzing the performance of the UHF RFID reader-writer.
Disclosure of Invention
Aiming at the problem that the strength of a backscatter signal cannot be directly measured due to aliasing of a self-interference signal with large amplitude and a backscatter signal with small amplitude, the invention provides a measuring method which can effectively measure the strength (amplitude) of the backscatter signal.
In order to achieve the above purposes, the technical scheme adopted by the invention is as follows:
a method for measuring the strength of a backscatter signal specifically comprises the following steps:
the method comprises the following steps: the reader-writer sends a continuous carrier signal to the label;
step two: the label receives the continuous carrier signal, and controls the backscattering of the continuous carrier signal according to the modulation signal p (t) of the label to obtain a label backscattering signal;
step three: the reader-writer is used for receiving the tag backscatter signals and measuring the frequency spectrum of the tag backscatter signals;
step four: and according to the frequency spectrum measurement result of the reader-writer, the amplitude of the tag backscatter signal is solved.
On the basis of the above scheme, the reader/writer includes: the device comprises a radio frequency unit and a baseband signal processing unit;
the radio frequency unit is used for sending continuous carrier signals and receiving label backscattering signals;
the baseband signal processing unit is used for measuring the frequency spectrum of the tag backscatter signal.
On the basis of the scheme, the label is internally provided with a micro-processing unit.
On the basis of the scheme, the continuous carrier signal t sent by the reader-writer in the step one R (t) representsComprises the following steps:
t R (t)＝Acos(ω 0 t) (1)
wherein A is continuous carrier signal strength, omega 0 Is the angular frequency of the continuous carrier signal;
on the basis of the scheme, the modulation signal p (t) of the tag in the second step is a periodic signal, including a common periodic square wave signal.
On the basis of the above scheme, the modulation signal p (t) is represented as:
wherein p is 0 (t-nT s ) Is the square wave signal of the nth period, n is a label, tau is the pulse width of the square wave signal, T s Is the period of the square-wave signal, p 0 (t) is a square wave signal of one period.
On the basis of the scheme, the label backscatter signal t in the step two T (t) is expressed as:
t T (t)＝p(t)·|Γ|·ρ·Acos(ω 0 t+θ 1 (t))+n(t) (4)
wherein, Γ is the reflection coefficient of the label, p (t) is the modulation signal of the label, ρ is the path loss from the reader-writer to the label, and θ 1 (t) is the phase difference, n (t) is noise, which is generally small and negligible,. Omega. 0 Is the angular frequency of the continuous carrier signal.
On the basis of the above scheme, the backscatter signal received by the reader-writer in the third step is:
r R (t)＝p(t)·B 1 ·cos(ω 0 t+θ 2 (t))+B 2 ·cos(ω 0 t+θ 3 (t)) (5)
wherein p (t) is the modulation signal of the label, B 1 For the backscatter signals received by the reader-writerIntensity, expressed as: b is 1 β = β · | Γ |. ρ · a, β is the tag-to-reader path loss, B 2 For self-interference signal strength, θ 2 (t)、θ 3 (t) is a phase difference.
Since the tag backscatter signal experiences dual free-space fading, usually B 1 ＜＜B 2 。
As can be seen from the formula (5), the signal received by the reader is the superposition of the self-interference signal and the tag backscatter signal, and the center frequencies of the self-interference signal and the tag backscatter signal are both f 0 ，
And the self-interference signal is much larger than the tag backscatter signal. The spectrum of the self-interference signal and the tag backscatter signal is at f 0 The frequency is superposed to form a peak, and the backscattering signal of the label at f cannot be directly observed 0 The power value at frequency.
On the basis of the scheme, the fourth step specifically comprises the following steps:
the tag backscatter signal is equal to the modulation signal of the tag multiplied by the continuous carrier signal received by the tag, and according to the nature of the fourier series, the time domain product is equivalent to a spectral convolution:
continuous carrier signal t R (t) the corresponding spectral formula is expressed as:
F[ω]＝π[δ(ω-ω 0 )+δ(ω+ω 0 )] (6)
where ω is the angular frequency, δ is the unit impulse function, ω is 0 For the angular frequency of the continuous carrier signal, delta (omega-omega) 0 )、δ(ω+ω 0 ) Respectively represent. + -. ω 0 Unit impulse function of pi [ delta (omega-omega) ] 0 )+δ(ω+ω 0 )]Representing the sum of two unit impulse functions with amplitude pi;
the spectrum formula corresponding to the modulation signal p (t) of the tag is expressed as:
wherein the content of the first and second substances,
as a function of the number of the coefficients,
ω s representing the angular frequency of the square wave signal,
represent
Value of sinc function, delta (omega-n omega) s ) Represents the nth ω s A unit impulse function, n is a label;
tag backscatter signal t T (t) the corresponding spectral formula is expressed as:
wherein E = | Γ |. ρ · A, E is the tag backscatter signal strength, Γ is the tag backscatter coefficient, ρ is the reader-to-tag path loss, A is the transmitted continuous carrier signal strength, ω is s Representing the angular frequency, omega, of a square-wave signal 0 At the angular frequency of the continuous carrier signal, delta ((omega-n omega) s )-ω 0 )、δ((ω-nω s )+ω 0 ) Respectively represent (omega-n omega) s )±ω 0 A unit impulse function, n is a label;
tag backscatter signal at omega 0 And omega 0 +nω s The amplitude ratio at the angular frequency is 1
The power ratio is
Obtaining a backscatter signal at omega by spectral measurement 0 +nω s Angular frequencyPower value at rate, according to ω 0 And omega 0 +nω s Power ratio of the position, derived from the backscatter signal at ω 0 The power value at the frequency, and thus the amplitude of the backscatter signal.
The technical scheme of the invention has the following beneficial effects:
by adopting the method for measuring the strength of the backscatter signal provided by the invention, the strength (amplitude) of the backscatter signal of the tag can be accurately obtained through simple measurement and theoretical calculation, the implementation is simple, the calculation amount is small, and the method has important significance for analyzing the performance of an RFID reader-writer.
In addition, the measuring method provided by the invention can be used for estimating the signal strength of an uplink (from Tag to Reader) in the RFID system, estimating a channel in real time and adaptively adjusting a sending signal, thereby reducing the error rate and having an auxiliary effect on improving the performance of the RFID system.
Drawings
The invention has the following drawings:
fig. 1 is a block diagram of a backscatter communication system architecture.
Fig. 2 is a schematic diagram of the frequency spectrum of the modulated signal of the tag.
Fig. 3 is a schematic diagram of a tag backscatter signal spectrum.
Fig. 4 is a first connection block diagram of the backscatter signal strength measurement system.
Fig. 5 is a connection block diagram of a backscatter signal strength measurement system two.
Fig. 6 is a schematic diagram of a power spectrum of a signal received by the reader/writer.
Fig. 7 is a graph showing the amplitude ratio of the self-interference signal to the tag backscatter signal at different distances from the reader antenna and at a circulator isolation of about 22 dB.
Fig. 8 is a block flow diagram of the present invention.
Detailed Description
The present invention is further illustrated by the following examples in conjunction with figures 2-8, it being understood that these examples are intended only to illustrate the invention and not to limit the scope of the invention, which is defined in the claims appended hereto, and that modifications of various equivalent forms to the present invention by those skilled in the art will be within the scope of the present invention after reading the present invention.
A backscatter signal strength measurement system as shown in figure 4 comprising:
in order to facilitate the measurement of the frequency spectrum of the tag backscatter signal, in the embodiment of the application, a frequency spectrograph is added.
A PC, a Universal Software Radio Peripheral (USRP), a circulator, an antenna, a frequency spectrograph and a label;
after the PC is connected with the USRP, the PC is connected with an antenna through a circulator, the antenna is connected with a frequency spectrograph through the circulator, and the label is communicated with the antenna;
the PC is connected with the USRP to realize the function of a reader-writer and is used for generating a continuous carrier signal with the center frequency of 915 MHz; the circulator is used for isolating the transceiving link; the label is used for generating periodic square wave signals and controlling backscattering of continuous carrier signals; the antenna is used for transmitting and receiving signals; the frequency spectrograph is used for measuring the power value of the signals received by the reader-writer.
The working principle of the backscatter signal strength measurement system is as follows:
the PC machine and the USRP realize the function of a reader-writer, generate radio-frequency signals, transmit the radio-frequency signals through the port 1 to the port 2 of the circulator through the antenna, generate periodic square wave signals by the label, control the reflection of continuous carrier signals, namely high level reflection, and obtain backscatter signals by not reflecting low level. The antenna receives the backscattering signal and enters the frequency spectrograph for measurement through ports 2 to 3 of the circulator. The waveform observed on the frequency spectrograph is a mixed power spectrum of the self-interference signal and the received tag useful signal from ports 1 to 3 of the circulator.
A backscatter signal strength measurement system may further comprise, as shown in fig. 5:
the system comprises a PC, a USRP, a transmitting antenna, a receiving antenna, a frequency spectrograph and a label;
the PC is connected with the USRP, is used for realizing the function of a reader-writer, and then is connected with a transmitting antenna, and the receiving antenna is connected with a frequency spectrograph;
the PC machine + USRP is used for generating a continuous carrier signal with the center frequency of 915 MHz; the label is used for generating periodic square wave signals and controlling the reflection of continuous carrier signals; the transmitting antenna is used for transmitting signals, and the receiving antenna is used for receiving signals; the frequency spectrograph is used for measuring the power value of the signals received by the reader-writer.
The working principle of the backscatter signal strength measurement system is as follows:
the distance between the PC + USRP and the frequency spectrograph is d1, and the distances between the PC + USRP and the label are equal to d2, so that the free space loss of the transceiving link is the same. The radio frequency signal generated by the PC and the USRP is sent out through the sending antenna, the signal reflected by the label is received by the receiving antenna, and the signal is measured through the frequency spectrograph. The waveform observed on the spectrometer is a mixed power spectrum of the self-interference signal passing through d1 and the useful signal reflected by the tag.
The method for measuring the strength of the label backscatter signal by adopting the scheme comprises the following steps:
the method comprises the following steps: the reader-writer sends a continuous carrier signal to the label;
step two: the label receives the continuous carrier signal, and controls the backscattering of the continuous carrier signal according to the modulation signal p (t) of the label to obtain a label backscattering signal;
step three: the reader-writer is used for receiving the label backscatter signals, and the frequency spectrograph is used for measuring the power value of the label backscatter signals;
step four: and solving the amplitude of the tag backscatter signal according to the result of the spectrometer measurement.
In this embodiment, the continuous carrier signal power emitted by the reader/writer is about 21.8dBm, the circulator isolation is about 22dB, and the tag generates a periodic square wave duty cycle of 1/2.
Specifically, in the third step, the power spectrum of the signal received by the reader/writer end is as shown in fig. 6, where ω is 0 The value at the angular frequency is the sum of the power of the tag backscatter signal and the self-interference signal, the ratio of the tag backscatter signal power at 1 and 2 is
The power value at the position 2 can be directly read by a frequency spectrograph, and the power value at the position 1 can be calculated, so that the amplitude value can be obtained.
In the measurement process, the distance between the tag and the antenna is changed, the power of a main peak and the power of other peaks measured by the frequency spectrograph are recorded, and a group of test results are shown in table 1, wherein N/A indicates that the power value is too small, and a signal is submerged in the bottom noise and corresponds to the zero position in the sinc function.
TABLE 1 Spectroscopy test results
The data in table 1 is analyzed according to the method, the power values of the first side lobe and the third side lobe are taken to calculate the power value of the main peak, the calculation results of the two times are averaged to obtain the calculation result shown in table 2, and meanwhile, the line graph of the amplitude ratio of the self-interference signal and the label backscatter signal is shown in fig. 7.
It can be seen that the farther the distance between the tag and the antenna is, the smaller the value of the power of the backscatter signal is, and the larger the ratio of the power and the amplitude of the self-interference signal to the backscatter signal is. When the distance exceeds 190cm, the side lobe in the actually measured frequency spectrum is submerged by the bottom noise, the value cannot be read continuously, the amplitude ratio of the self-interference signal to the backscattering signal is 29.6dB and still less than 42dB, and the backscattering signal can be judged.
TABLE 2 tag backscatter signal strength
The technical key points and points to be protected of the invention are as follows:
in a backscattering (including RFID) communication system, for strong self-interference signals and weak Tag backscattering signals with frequency domain aliasing, periodic signals are generated by controlling tags, wherein the periodic signals can be square wave signals, so that the frequency spectrum of the Tag backscattering signals accords with a certain rule, the amplitude of the Tag backscattering signals at the central frequency is obtained by utilizing the amplitude of side lobes, and then the amplitude of the backscattering signals is obtained.
References (e.g. patents/papers/standards)
[1] Yuan Yong, yu Dan, lanpu Marthels, etc. self-interference signal cancellation apparatus and method and radio frequency identification reader/writer, china, 200710167268.4[ p ],2009-05-06.
[2] Wang Gongpu, xiong Ke, liu Ming, high fly, zhong Zhangdui backscattering communications & internet of things [ J ] academic news of internet of things, 2017,1 (1): 67-75.
Those not described in detail in this specification are well within the skill of the art.
Claims (3)
1. A method for measuring the strength of a backscatter signal, comprising the steps of:
the method comprises the following steps: the reader-writer sends a continuous carrier signal to the label;
step two: the label receives the continuous carrier signal, and controls the backscattering of the continuous carrier signal according to the modulation signal p (t) of the label to obtain a label backscattering signal;
step three: the reader-writer receives the tag backscatter signals and measures the frequency spectrum of the tag backscatter signals;
step four: according to the frequency spectrum measurement result of the reader-writer, the amplitude of the tag backscatter signal is solved;
step one continuous carrier signal t sent by the reader-writer R (t) is expressed as:
t R (t)＝Acos(ω 0 t) (1)
where A is the continuous carrier signal strength, ω 0 Is the angular frequency of the continuous carrier signal;
step two, the modulation signal p (t) of the tag is a periodic signal, including a periodic square wave signal;
the modulation signal p (t) is represented as:
wherein p is 0 (t-nT s ) Is the square wave signal of the nth period, n is a label, tau is the pulse width of the square wave signal, T s Is the period of the square-wave signal, p 0 (t) is a periodic square wave signal;
step two, the label backscatter signal t T (t) is expressed as:
t T (t)＝p(t)·|Γ|·ρ·Acos(ω 0 t+θ 1 (t))+n(t) (4)
wherein, Γ is the reflection coefficient of the label, p (t) is the modulation signal of the label, ρ is the path loss from the reader-writer to the label, and θ 1 (t) is the phase difference, n (t) is noise, negligible, ω 0 Is the angular frequency of the continuous carrier signal;
step three, the backscatter signals received by the reader-writer are:
r R (t)＝p(t)·B 1 ·cos(ω 0 t+θ 2 (t))+B 2 ·cos(ω 0 t+θ 3 (t)) (5)
wherein p (t) is the modulation signal of the label, B 1 The strength of the backscatter signal received for the reader is expressed as: b 1 β = β · | Γ |. ρ · a, β is the tag-to-reader path loss, B 2 For self-interference signal strength, θ 2 (t)、θ 3 (t) is the phase difference;
since the tag backscatter signal experiences dual free-space fading, B 1 ＜＜B 2 ；
According to the formula (5), the signals received by the reader-writer are the superposition of self-interference signals and label backscattering signals, and the self-interference signals and the label backscattering signals are obtainedThe center frequencies of the interference signal and the tag backscatter signal are both f 0 ，
The fourth step specifically comprises:
the tag backscatter signal is equal to the modulation signal of the tag multiplied by the continuous carrier signal received by the tag, and according to the nature of the fourier series, the time domain product is equivalent to a spectral convolution:
continuous carrier signal t R (t) the corresponding spectral formula is expressed as:
F[ω]＝π[δ(ω-ω 0 )+δ(ω+ω 0 )] (6)
where ω is angular frequency, δ is unit impulse function, ω 0 For the angular frequency of the continuous carrier signal, delta (omega-omega) 0 )、δ(ω+ω 0 ) Respectively represent +/-omega 0 Unit impulse function of pi [ delta (omega-omega) ] 0 )+δ(ω+ω 0 )]Representing the sum of two impulse functions with amplitude pi;
the spectrum formula corresponding to the modulation signal p (t) of the tag is expressed as follows:
wherein the content of the first and second substances,
as a function of the number of the coefficients,
ω s representing the angular frequency of the square wave signal,
to represent
Value of sinc function, delta (omega-n omega) s ) To representN th ω s A unit impulse function, n is a label;
tag backscatter signal t T (t) the corresponding spectral formula is expressed as:
wherein E = | Γ | ρ · A, E is the tag backscatter signal strength, Γ is the tag backscatter coefficient, ρ is the reader-to-tag path loss, A is the transmitted continuous carrier signal strength, ω is the tag backscatter signal strength, and s representing the angular frequency, omega, of a square-wave signal 0 At the angular frequency of the continuous carrier signal, delta ((omega-n omega) s )-ω 0 )、δ((ω-nω s )+ω 0 ) Respectively represent-n omega s ±ω 0 A unit impulse function, n is a label;
tag backscatter signal at omega 0 And omega 0 +nω s The amplitude ratio at the angular frequency is 1
The power ratio is
Obtaining a backscatter signal at omega by spectral measurement 0 +nω s Power value at angular frequency, according to ω 0 And omega 0 +nω s Power ratio of the position, derived from the backscatter signal at ω 0 The power value at the frequency, and thus the amplitude of the backscatter signal.
2. The backscatter signal strength measurement method of claim 1, wherein the reader comprises: the device comprises a radio frequency unit and a baseband signal processing unit;
the radio frequency unit is used for sending continuous carrier signals and receiving label backscattering signals;
the baseband signal processing unit is used for measuring the frequency spectrum of the tag backscatter signal.
3. The backscatter signal strength measurement method of claim 1 wherein the tag has a microprocessor unit built into it.
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| CN105550720A (en) * | 2015-11-27 | 2016-05-04 | 杨刚 | RFID system capable of enhancing read-write distance and read-write method thereof and energy distribution optimization method |
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